Keen, A. Myra

English

Aquatic Commons

VoL 3, No.9 OcTOBER 16, 1962 
A NEW WEST MEXICAN SUBGENUS AND NEW SPECIES 
OF MONTACUTIDAE (MOLLUSCA: PELECYPODA), 
WITH A LIST OF MOLLUSCA FROM 
BAHIA DE SAN QUINTIN 
By 
A. MYRA KEEK 
STANFOHD UNlVIHSJTYr CALlFOHNIA 
Among the mullu:-:;ks taken during an ecological sun·ey of Bahia de 
San Quintin, Baja California-- sent to me Ior (leterrnination hy members 
of the Beaudette Foundation I uoileed a single specimen of un unusual 
hivalvc. The sheH superficially resembled a Cardita~ save that il did nol 
actually haH· radial rih~ and there was no evident external ligament; 
within, the disjointed palHal line and the hinge implied relationship to the 
genus 1\.ellia in Leplonacea. Howen~r, the re~iliicr wa;:; of remarkable size 
and unusual shapc1 anrl the internal ligament! partially detaehed~ was so 
calcified as to resemhlc a lithodesrna~ hallmark of the Auomalodesmala; 
porcelanous. not nacreous, tihell material could not justify such a placement. 
Iu the end, allocation to Leptonacea, family 1\iontacutidae, seemed to be 
indicated, especially after I had had the privilege oi studying material from 
Ecuador loaned to me hy Dr. A. A. Ols.~ont which resolved .BOrne problems 
not cornpletPly covered in his discussion oi the genus Owbitella tOlsson~ 
1961, pp. 235-238). 
The most closely related gcmu!i' seems to he Orobiiella DaiJ~ 1900, hut 
so many points o£ difference are evident that the proposal of a new 
~mbgenus seems advisable. lVlost oi th0 named species of Orobitella are 
quadrate! not trigonal, iu outline, with a le~s strongly developed hinge. 
The genus is tropical American in distribution. ranging on the Atlantic 
side from late T Prtiary to B 0cent in Florida and on the Pacific Coast ftom 
Plioccm· to 1\ecent in Peru, northward at least to California. 
322 PAClflG NATUH.ALIS1' 
Family MONTACUTJDAE Clark, 1855 
Genus Orobitella Dull, 1900 
Trans, VVngne1· Free Inst. Sci." voL 3, pt. 5. p, "11 HJ. 
VoL. 3, No.9 
TYPE (mnnntypy): Jllonlacutri (0.) floridaua Dall, 1899. 
The lypc ;-;pecies 1 from '\Vcr-l Florida, is large (to 16 mm. in length), 
inequilatL·ral, the anlt•rior end longer than the po;;;terior, outline quadrate. 
As in all membcr8 oi the fumilr, there is 11 resiHary area under th("' hcuks. 
The uriginal figur<• ( Dall, 1899, p. 893~ pl. 87, fig. 10) shows Litis urea as 
shorl. A lruein~; of the figure is gin•n here (fig. 2). Aeeonling- lo DaH's 
deseriplion, there is a cardinal loolh or hook in either valve in front of 
the re3ilium. 
Olsson and Harhison (I 953, p. 96) 1mmed a smaller subspecies of 
0. florldarm (length 11 mm. I, r rom the Floddu PHoeene, and ngured the 
hinge of tbe ldt valve~ which Fohow::; a moderately wifh~ plat(: ohliquely 
cut by the rcsilifer. a triangular gap or boss lwing evident between rcsilifer 
and cardinal (fig. 3). Olsoon (1961. pp. 235-238) has described and 
figun~d s::.everal form~ from northt•rn Peru thal exhihit th(' same features. 
He hus kindly loaned me specimens of these for sludy. Some specimens 
have a thin~ radially shaggy pcriostracum, hut most forms are smootht 
The ane-eslral stock of Orobitello prohahly is the i\:lioeene Neaeromj'a 
Gahh, 1873 (Proc. Acari. Nat. Sci. Philadelphia lor 1872, p. 274; type 
species hy monotypy. N. quudrota Cabb~ 1873). Because of Gahh's erron· 
cous statement that the hinge hears lateral teeth: Dall \\'US misled as to the 
alfinitics of Neaerom.ya. His suhgenus Orob£tr:lla adually :is closer to 
Ncueromya than to 11lonlacula, and if it is not now accorded generic rank 
should he made a suhgPnus of Neaeromya. fn a re-stmly nJ Gabb;s type 
specimens, Pilsbry (1921, p. 419, text~ fig. :I.S) has shown that the hinge 
plate is smooth in Ncaeromya except for one cardinal hook in the right 
vain·. The lack of a tooth In the ldt valve thus separates it from Orobitelta. 
Pilsbry's figures are n~produced here, in Jnc"imile (figs. la_.ld). Neacromya 
is restricted to lhe l\Hocenc o[ Jamaica, Santo Domingo [Dominican Re· 
public], ami Florida. 
A possihie European rmalognc of Neaerom;·a and Orobitcllu may be 
the Tertiary gPmls Pleurorlcsma. HOmes, 1859. It has a single laminar 
cardinal in either valve and an C'"XCa\'atr-d rcsifift•r unrler the beaks; the 
outline is lrapezoidal to quadrate. Cassmann and Peyrot in 1909 erected 
a separate family for it and allocated it to 1Iyacca, however. A restudy 
of specimen~S Irom the N!iocene oi France nnd Austria would be desirable. 
A Soulh Ameriean genus somewhat similar in outline to Orobitella is 
Diplodontinu Stempel\, 1899 (Zoo!. Jahrb. Suppl. Brl. 5, Fauna Chilensis 
2. p. 232), from Chile. As the figure of the hinge shows two cardinal teeth, 
this mny belong in Kelliidae rather than in 1\.'Iontacutidae. 
Olseon (1961, p. 236) considers tbat the shell deserihed by Dall as 
Sportelltt sleamsi, from the Gulf ol California (Dull, 1899, p. 885) is an 
1962 KEEN: PELECYPODA: OJWBITELLA 323 
Orobitella, which seems plausible. This being granted, one must wonder 
whether other similar forms presently allocated lo Spur/ella and Pseudopy-
thina would not also qualify as Orobitella and thus extend the range of the 
gt~nus northward to California or even~ perhaps, Lo Alaska. 
Subgenus Isorobitella Keen, subgen. nov. 
TYPE SPECIES: Orobitella (Isorobitella) singularis, sp. mn'. 
DESCHIPTIO~: <halt•-trigotwl, height and length nearly equal; shell 
malt•rial relatively slunly, porcelanotJs, slightly translucent; perinstracurn 
well deYelnped, larger than shelL free at nmrgins amL in drying, overlap-
ping the inner margin of shell, shaggy in ll':xlure, Llw tufts or hri.o:Lles laid 
on in fine radial rows; excavation for re..-ilium with a nwtlian ridge. mar-
gins slightly upturned; resilimn of three layers, the outer and imwrmost 
non-calcified, the medial lap'r whili."h, infiltrated with fibrow:: needles of 
calcite; adductor nmsde scars conspicuous, ovate, lnrge; pallial line wille, 
well removed from ventral margin, of irregular but nearly merging 
attachment scars. 
REl\TAI{KS: The ..-ullgeneric name adds the prefix. isos, from the Greek 
word for like, to Orobitella. Allhough Dall gave no derivation for Orobitf'lfa, 
it evidently is coined from orobus, name of a plant with a bean-shaped seed. 
!.wrohitdla differs from Orobitella hy having a differently-orienterl 
resilium, which parallels the hinge margin, with a median ridge that is 
complete lacking eilher in the speeimens kindly loaned to nw by Dr. Olsson 
or in any of the figures yet puhlisherl. Tn lsorobitella then' is no gap he-
tween re.':dlifer and cardinal. Attachment scars of the rt>silifer in Orobitelfa 
show only a single cune, whereas in l.mrobitella they are douiJly sinuous. 
Also, tiil' shell is distinctly more com·ex in lsurobitelfa. at least in the single 
-"JWcimen available for ('omparison. The periostracum is he<wier" larger, and 
more sh:!ggy than in Orubitella; many specimens of the latter show no 
pi'riostracnm at all. 
Orobitella (Isorobitella) singularis I\..een, sp. nm·. 
Figures 4, a-c, 5 a-h 
HoLOTYPE: Stanford Univ. Pale:J. Type Coli. no. 9518. 
TYPE LOCALlTY: Bahia de San Quintin, Baja California del Norlt>, 
Baja California, lVIexico; mud flats on northeast part of llay. (\Vest Coast 
of Lower California, approximately :30° 28' N. Lat., 115° 2W \V. Long., 
O.G milrs ESE of cannery, near benthic biological station no. 15). Collec-
tors, J. L. Barnard and P. T. Beaudette, ApriL 1961. 
DDIENSIONS OF HOLOTYPE: Length, 9 . .5 mm.; height. 8.8 nun.; 
diametPr (lmth valves), S.d mm. 
DESCRIPTION: SlH'll ovate-trigonal, umbones wide" heaks small, low; 
exterior surface CO\'l'red by a shaggy hrown perinstracum regularly ruisell 
into shreds or tufts in about GO fine radial rows, giYing the illusion of radial 
sculpture; periostraeum continued beyond shell margin, folding over 
32'1< PAClFlC NATURALIST VoL. 3, No. 9 
I b 
Ia 
I c 
ld 
Fig. I. Neaeromra quadrata Gabb. After Pilshry, 1922, rigs. 45 a-d. la, interior 
of holotype, right valve, x 7; lh, dorsal view, holotype; lc enlarged 
\--ie\v of hinge, x 20; IlL interior of a smaller pnratype, )eft valve, x 7. 
Le-ngth of holotype, 6.2 mm, l'viiocene, 'VVest Indies. 
Fig. 2. Orobitella (Orobitclla) /lorirlana (Dall). After Dall, 1899, pl. 87, fig. 
10 (modified tracing). Holotype, right valve. x 2.5. Length, 16 ntnt. 
fk-ccmt, "''estern Florida. 
Fig. 3. 0. (0.) floridanu inflala Olsson and Harbison, 1953, Tracing from origi-
nal figure (pl. 9, fig. 6). Holot:rpe. 1eft valve, X 12. Length, 10.7 mm., 
height, ti5 mm., diametet' (one vahe), 2.2 nmt. Pliocene, Florida. 
1962 KEEN: PELECYPOD:\: OtWBITELLA 325 
ventral margin in holotype; surface of the white ~hell apparently smooth 
except for growth lines, with a low angle at the boundary of central 
and anterior slopes; hinge with one peg-shaped cardinal lamina in either 
valve and no lateral tcdh; ligament and resilium fused~ long, internal, 
with three distinct layers, the outer (or upper) dark IJrown, the medial 
whitish (due to intercalated Iihers or needles of caleitic material), the 
innermost thin, brown, nearly tnmsparent, lodged in an excavation formed 
by the broadening of the hinge plate }Jack of and below the beaks, 
apparently somewhat larger in the left valve than in the right and divided 
into two parallel parts by a median ridge, with one part facing the opposite 
valve, the other facing into the shell cavity; edges of the resilifer slightly 
raised or even somewhat laminar; muscle scars large, elliptical, the anterior 
scar longer, its inner edge and the continuing upper border of the pallial 
line ragged or irregular; pallial line well removed from ventral margin; 
inner margin of shell smooth. 
DERIVATION OF NA)IE: The Latin adjective singularis, alone or soli-
tary, seems an appropriate term for this bivalve, for which additional 
searching hrought no further material to light. 
RE~'IAI{KS: With only a single specimen at hand, one cannot be cer-
tain of the degree to which observed features may represent individual 
variations. However, the consistency of hinge features in Orobitella., s. s., 
lends strength to the conviction that the diiierences noted here arc reason-
ably stable. 
The literature on ligamenta} structures in the Hcterodonta is scanty. 
Calcification of some part of it is not common but has been mentioned by 
Dall (1900, p. 1170), in discussion of Lcptonacea, but he is careful to 
state that this does not form a lithodesma, which, of course) is correct. A 
ligamenta} capsule similar in structure to the one in lsorobitella is oLsen'· 
able also in Kellin luperousit (Desha yes), from California, in family 
Kelliidae. The resemhlan~e to a lithodesma is shown in fig. 4c, in a camera 
Iucida drawing of the right valve of 0. (/.) 8ingulari.~, just as il appears, 
with the inner layer of calcified ligament, torn from its socket in the left 
valve when the shell was opened, showing as a roumled structure beneath 
the hinge, much in the position of a true lithodesma. However, in life this 
was a compressed sheet or mass of resilium in the now-empty cavity of 
the left dorsal margin; drying has ~a used it to curl, and the white medial 
layer takes on the appearance of a separate glohular structure~ the trans-
parent conchiolin of the innermost layer not being evident. 
Fig. ·k Orbitclla (Isorobitclla) singularis Keen, sp. no\'. Holotype, Stanford 
Univ. Paleo. Type Coll. no. 9518. 4n, Hinge, left Ynlve, x 10, normal 
orientation; 4b, same ynlve, tilted backward to shovv inner portion of 
resilifer and the medial ridge; +c, hinge, right valve, shmving outer aJI(I 
middle layers of ligament and the detached ligament from left. valve 
hanging below hinge, simulating n lithodcsma. x 10. Recent, \iV. I\1exico. 
326 PACIFIC NATUILU~IST VoL. 3, i\o. 9 
Fig. :i. Orobitelln ( lsorobitclla) singularis Keen~ !ifl, 11nv. ffolat:>'P'~' SU uo, 9518. 
5<t. Extnriol', right valve, x 5; 5b. interior, 1t•ft vnlnt, with radially tufted 
periostrncum ovPI'Iupping 1lm vnlve margins as an in·ogulnr dark border. 
Length, 9.:3 nnn. HecenL San Quintiu Bay, Bnja California. 
The rnatL·rial of which tlw shelJ i"' composed shows ftll unusual texture~ 
also. Above the pallial line and mu:::de sears the hmcr shell layer Is lhe 
normal whitish eolor~ inconspicuously pillet!, hut those parts of the shell 
where the mantle and adductor rnu~clcs wt·rc attached has a glazed or 
translucent lextun•, of a hluish-grny color. Fortuna1Ply. lht• spedmeu was 
pn:st~rved in alcohol ht"'lon~ drying. ThiF leave:- lhe tisfiues intact. attd 
g;ros:' fentun:s can hP sf~Pn. The ctl'nidiurn i:; well pn·;,erved~ reticulate in 
form. with sewrnl plater<, 
Onl' may f.:JWcnlate a:- to the fuliction of tlw m-t·r~f'Jzcd pcriostracum 
and the exnggerall',dly large hlnf!ing npparatu.:; (hath ligament and must:ul-
ature). :May H he lhal in life lhc animal lies with lhc valves slightly 
open {a~ in sumt~ uf the Galeommalid::"), anrt that the large hinge is needed 
tn cnnlml this form of gape? Tlw pn'dse habitat remains to lw. dh:cow:red. 
In Brilaln memher~ of th0 1\'Iontacuti{hte are commensal with Echinoder-
mata. cnpeeiaHy hi.,cuit urchins. Conunensalism is prohahJe lwrc, too. The 
:::o]e specilm:n wa~ found on a murl-flat -when search was being made for 
certain Cru:;lueea~ but laler seareh did not rcYcul il us being a!'sociated 
with them. Tht• domiunnt. mollu:-ean associate at the site wus Cryptomya 
tcalijornica Courml. 
A e-:Jmpletc li;;-1. of the Mollusca taken duri11g the ecological snney 
of San Qutnti11 Bny is gin~n hdow. 
1962 KEEl_\;": PELECYPO!JA: Otl.OllJTELLA 
LIVING MOLLCSCA COLLECTED DUUNG A~ ECOLOGICAL 
SURVEY OF BAHIA DE SA~ QUlNT!N 
327 
I ntPrtidal collections (especially washings from algae) and hay-floor 
s.amples taken with an orange·peel gruh are listed alphahetically here. 
Station numbers are indicated hy italics~ numher of :>pec-imens in ordinary 
type. Data for the station records are heing published separately hy Dr. 
Barnard, who al~o will giYe further information as to (•cology. 
Mollusks are poorJy n:pn•sented in this bay~ and most of the spL'cimens 
are immature. For !his reas.on, some identifieations an: open to que~tlon. 
8wveral o-f the mi11ute bh·alves could not be rletermined. 
Acmaea deplcta (Hinds) Si, 3. 
Actcocina carinata (Curperlter) 0, 12~ 1. 13: 2. 12: 3, 6; ·1. 7: 5, 1; 6. 1; 7. 
2; 12, 9: 15, 10,; 36, 1: 37, l; ·14. 6; ..f7, 3; 48, ·k 52. l; 56, L 59, 15; 63, 
·1•; 67, 5; 68, 14; 71. 3; i31 2; 75. 2; 80, 5; 84. l l; 85. t; S6. 2: SS, 9:91, 2.; 92, 
3, 93. L 
.4equipccten drcularis aequisulcatus {Cnrpenter) 79. 1. 
Assimim:a compacta (Carpenter), [Synccra Auctt.] 176. n 
Bittium quadr!fllatum Carpenter 10. L 
Brachidontcs sp. 175, 1. 
C}uwtopleura gem mea PHsln·:r 1 S 1, L 
Cooperella subdiaphana (Carpenter) 56, ! ; i3, L 
Crucibulum spinoswn Sowerhy 169. 2: JS(. 2·1 [forma tubiferum]. 
Cryptomyn cnlifornica Conrad (numerous ln nwd-flat probes). 
Haminoea sp. 67. 8. 
Hiatella ar-ctica (Linnneus) ?52. 1; 163~ 5; 175. 9: 179, 3. 
Lasaea subuiridis Dull .16_9. :5:5; 176, 5. 
LefJIOpectm lntiauratus (Conrad), var., approaching L. tumbe:::.cnsis {Orbigny) 
87, 1. 
Ly-onsia cali{ornica Conrnd 3S, 3; 4-!, 3:48, 1; 6.'5. 5; 69,2: 73, 16; 77, 2. 
Alncoma yoldi[ormi:; Cm1Jenter 65, 2; 69, L 
ii1arginella calif arnica Tomlin 16·11 +: 166, ·t~ 181, 19. 
1Harginella p)'r-iformis Carpenter .!i5, L 
"Harginella regularis Carpenter 166, 1; .175, 3; 181, 2. 
Jlditrella carinala (Hinds) ?57, t; 61, 3; 73, 1~ 81, 2, 
lVlodiolus ;;p •• all juveniles 3·1, 3; 4.J, 6: 60, 1: 61, t2; 6.J, 1; 65, 1; 66, 1; 72, 3~ 
73, 1, 79, 3, 83. 1, 92, L 
Odostomia (llienestho) fctella Dull rmd Bartsch 169_, 3. 
Odostomia sp. 48, 1. 
Ostrea lu.ritla Carpenter 131. 50+, 
Parvilucina tenuisculpta (Ca11H.mter) 4S, 2.; ?61. 3. 
Pratotlutca staminea {Conrad) Mudflnt, L 
Solen rosaceus Carpenter 1 ·1. 3; 17, 1 ; 21. I; 23, I; 36, 1; 64-, 1; tl5. '[; 69. 1 ; 
78. L 90, I; 9-1, 2. 
~28 PACIFI(: NATUllALTST VoL. :1, No. 9 
1'dlina builoni (Dall) 39, 1; .f.J. 21; 4S, 1~ 54, 3; 56_. 1; 60, 2; 65. 3; 72, 3~ 73, 
3; 77, 1; 81, 6; 92, g, 93. 5. 
1'crebra pedroana Dull 59, L 
Trachycardi.um qumlragenarium (Conrad) 73, 1; 77, L 
Transenru:lla tnntiUa (Goulrl) 67. so-t-: 71. 50+ [all dead but fresh shells]. 
?Tricolia sp. 62, 6. 
1'urbonilla sp. 3.1. 6; ·fi. L 
Nudi.hranchintn, 2 species 20, 2; 26. ·1: JS. 6: ·fD, 1: 43, 12.; .J9. t: 60. 1; 65, 2; 
66, l.; 67, 2; 71, 3~ J.l. !; 76. r!; 77. 5; 88, 1; 91, 1; 93.1. 
Indeterminate (limpets, shells dissolvnd by prcservntirx~) 34, 1: 61, 3: Sf, L 
h1d-ctenninnte (shells dissolved) 3,J. 1; 51. I; 53, 2: 61. 3; 74. ·I-; 75. 1: 76, 1: st~ 
I; SJ, 3. 
Acknowledgments 
1 wi:;,h to express my thank::; to lhe stafl of lhe Beaudette Foundation 
for the privilege of examining Lh('ir material and publis-hing the new 
record. The Held collections were supporled hy National Science Foundation 
grant G-11413. I am indebted to Monsieur Andre Chavan for advice as to 
the proper allocation or lhL~ pelecypod, to Dr. Axel A. Ol.s.son £or the loan 
of mmparutive material, and to Dr. \V. P. \Voodring for a report on the 
present stahli' of some of Dr. Dull's material. Illustrations herein are Lhe 
work of Mr. Perfcetu Mary~ technical assistant at Stanford University. 
References Cited 
DniL W. H., 
l89fL Synopsis of lhe Recent nml Tertiary Lcptonaccn of North America 
and the 'West Indies:, Proc. U, S. Nat. :c\'lus.. 2l: 873-897, pls, 87-88 
(June). 
1900. Tertlru·y Fuuna of I!lorkla. Trans. 'iVngner Fn~c lust. Sd. 3 (5): 
f}4.gw!2(H, pls. (Dec.). 
Olssou, A. A. and Anne Harbison, 
1953. Pliocene Mollusca of southern Floridn. Acad. NaL Sci, Philudeljlhia, 
Monog. 8, 457 pp., 65 pls. (NoY.). 
Olsson. A. A. 
1 96f. Mollusks of the Tropical Enstern P<ICific. Punnmic-Pacific Pelecypodn. 
Pnlcontologit::al Resenrch Institution, 574 pp., 86 pls. {lVtnr.). 
Piisbr:r, H. A., 
1922. Rev-ision o[ VV. ::\1.. Gnbb's Tertiary Molluscu of Santo Domingo. Proc. 
Acnd. Nnt. Sci. Philadelphiu, 73 (2}; 305-435, pls. 16-47 (Jan.). 


A new west Mexican subgenus and new species of Montacutidae (Mollusca: Pelecypoda) with a list of Mollusca from Bahia de San Quintin

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A new west Mexican subgenus and new species of Montacutidae (Mollusca: Pelecypoda) with a list of Mollusca from Bahia de San Quintin

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