Cope, E. D. (Edward Drinker), 1840-1897.

Whitfield, Robert Parr, 1828-1910.

American Museum of Natural History Scientific Publications

Article IV.-DESCRIPTION OF A NEW FORM OF MYA-LINA FROM THE COAL MEASURES OF TEXAS.By R. P. WHITFIELD.Among some invertebrate fossils presented to the Museumin I896 by Miss F. A. M. Hitchcock, Ph.D., there are sev-eral specimens of a Myalina so remarkably different from anyknown species of the genus that I venture to present it as anew form.Shells of the genus Myalina, although quite abundant inthe Coal Measures of the United States, are usually so mo-notonous in character, as regards their general form, that itis quite difficult to distinguish one species from another.There are about thirty species recognized in catalogues, butit is not easy to identify more than a few satisfactorily, un-less one considers the localities and the different geologicalhorizons at which they occur to be evidences of specific dif-ference. The interior of the shell also presents the samemonotonous features, even among the forms usually con-sidered to be distinct species; at any rate, this may be statedof the American specimens.In the original description of the genus, which was basedupon Upper Carboniferous shells from Vis6, Belgium, thevalves are stated to be equal, and most of the authors whohave considered the genus reiterate the statement, thoughMcCoy, King, and a few others have distinctly stated thatthe opposite is the case. In the American shells the inequal-ity of the valves is very pronounced. Another feature of theusual diagnosis is the assertion of the existence of an internalseptum within the beak of one or both valves. This featureI have never found in any American specimen. There issometimes seen in casts of Myalina angtlata Meek & Worthenfrom the Chester Limestone horizon a horizontal slit beneaththe beaks of specimens, where the substance of the shell hasbeen quite thin. This results from the cavity of the beakextending under the cartilage area, but that area is external[63]64 Bulletin American Museum of Natural History. [Vol. XVI,and not internal, and consequently the slit is not from aninternal septum, like that of Septifer or Dreissena, on whichthe anterior adductor muscle is situated. In our MyalinasFig. i.-Myalina copei, sp. nov. Exterior of right valve.this muscle is deeply sunken on the interior of the beak cavity.May not this be the case in the European shells also?The species now in question may be described as follows:Myalina copei, sp. nov.Shell rather large, somewhat ponderous, much thickened. Sub-triangular in general outline, the greatest convexity of the valvesbeing along the umbonal ridge. parallel to and a little within thebuccal border, whence the body of the valves slopes gradually to theposterior margin. Left valve larger, longer, and deeper than the op-1902.] Whitfield, New Form of Myalina. 65posite. Hinge-line long, straight, or slightly arcuate; posteriorextremity of the hinge extending beyond the body of the shell andforming a large posterior wing, which is more or less sharply roundedat the end; between the wing and the body of the shell the margin israther deeply and broadly sinuate. Basal line short, rather narrowlyrounded. Beaks slightly twisted in front and characterized by alobe-like protuberance of moderate size between the beak and thebyssal area. Ligamental area broad, longitudinally grooved by thelines for the ligament. From under the beak of the left valve a doubleligamental groove extends to the interior of the valve, while the liga-mental markings extend across it and down the anterior side of theIFig. 2.-MyaznXa cofei, sp. nov. Interior of left valve.shell to the byssal opening, which is long, but not conspicuous. Rightvalve smaller and shallower than the left valve, and marked with aligamental ridge corresponding with the groove in the beak of the leftvalve.Exterior surface of the shell strongly lamellose with heavy lines ofgrowth. Interior of each valve provided with a large, elongated pos-terior muscular impression; anterior muscular impression faintly[7anuary, o902.] 5,pv,",Ik0....I.I1.16, ..k -_1j.1J66 Bulletin American Museum of Natural History. [Vol. XVI.]outlined; pallial line distinct, nearly marginal along the byssalborder. Cavity of the shell deep, substance very thick.Formation and locality.-From the Coal Measures on the McKenzietrail near Fort Griffin, Texas. Originally from collections made byProfessor Edward D. Cope.

New form of Myalina

Abstract

Similar works

Full text

Available Versions

American Museum of Natural History Scientific Publications