Fourty one bottom sediment samples for mechanical and faunal analyses were collected from the western part of the Chijiwa Bay which covers an about 60 sq. km area off Mogi, Nagasaki City. The bottom sediments were divided into four sediment types and their areal distributions are shown in figure 5. A narrow belt of sandy silt classed as Type III is found in the western half of the shllow submarine valley and separates the near- and offshore sandy sediments classed as Type II or IIIa. Fourty two pelecypod molluscan species which are listed in Table 2 were discriminated from the unit sediment samples (200cc wet volume sediment) and eight predominant species were selected as shown in Table 3. The dominant molluscan remains were found in the silty sand of Type IIIa which represents a transition between muddy and sandy sediments
