LAMELLIBRANCHIATA. 



05 



These and other particulars are noted in the description of 

 fossil shells, and, when their inner or nacreous layer is pre- 

 served, its impressions reveal the organization of the ancient 

 fabricator and occupant as truly as do the processes and 

 joint-surfaces of fossil bones that of the extinct vertebrate 

 animal. To aid the young Palaeontologist in acquiring this 

 essential knowledge for the successful study of fossil Bivalves 

 the chief impressions are named in the subjoined figure of the 

 internal surface of the left valve of the shell of a Cythercea 

 (fig. 16). When two adductors are present, as in the bi- 

 valves thence called " dimyary," they leave the " anterior" and 

 ''posterior muscular impressions ;" when one adductor only 

 exists, it answers to the posterior muscle, but is more central 

 in position (fig. 17, 0- The oyster is a familiar example of such 

 "monomyary" bivalve. When the "pallial line" or "impres- 

 sion" extends in an unbroken curve from the anterior to the 

 posterior muscular impressions (fig. 20, 4), it may be inferred 



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that the inhabitant of the 

 shell had either no siphon, 

 or a very small or a non- 

 retractile siphon ; when 

 the line is bent towards 

 the centre before it reaches 

 the posterior adductor 

 (figs. 16, 20, 8), the pre- 

 sence of a retractile siphon 

 is indicated, this notch 

 being occupied by the re- 

 tractor muscle of that part. 



TTT1 /> , • Cytherea chione, left valve. 



When a foot is present, 9 J 



its retractor-muscles usually leave recognisable marks on the 

 interior of each valve. The siphons in some of the elongated 

 Inclusa cannot be retracted into the shell ; they are conse- 

 quently exposed, as in Psammobia, fig. 14, g, g\ and Pliolas; 



F 



