198 SYNOPTIC COLLECTION. 



specialized through the habit of boring, etc., so that it 

 would seem as if these were reduced rather than primi- 

 tive forms of the class. It is probable, however, that they 

 are simply reduced members of the order which has Sol- 

 enomya and Anatina for its primitive representatives. 

 The weak, toothless condition of the ancestors could 

 hardly be preserved in the descendants, as pointed out by 

 Dall, unless the animals became borers or burrowers. 



If the shell-bearing ancestral form of the Mollusca is 

 sought in the Cambrian formations, one finds that nearly 

 four hundred species of molluscs then existed which 

 include representatives of nearly all the great orders exist- 

 ing to-day, and which, according to Cooke, 1 are without 

 the slightest sign of approximation to one another. If 

 this is true, the point of convergence of these divergent 

 lines lies far back in pre-Cambrian times. Until more 

 investigations have been made on these ancient rocks, 

 one can judge of the ancestral forms of Pelecypods suc- 

 ceeding the plate or cap-like condition by inference only. 

 It seems probable, however, that such a form possessed a 

 small, smooth, more or less circular shell ; that the two 

 valves were equal in size and were connected at the tooth- 

 less hinge area by a flexible membrane, the ligament. 

 This is the character of the young shell or prodissoconch 

 (Jackson) of many larval bivalves existing to-day. Such 

 a form may be represented by Modioloides prisca Walcott 

 (No. 369, fig. i, enlarged), found as an internal cast in 

 the Cambrian formation. Another genus, Cardiola (No. 

 369, fig. 2, C. cornucopiae Goldf.), possesses most of the 

 archetypal characters. 



The descendants of such a form may be Solenomya, 

 Anatina, and the like. Solenomya velum Say (No. 370 ; 

 No. 371, shells), has a small, thin, delicate shell, having, 

 contrary to rule, the posterior end shorter than the ante- 



1 Cambridge Natural History, Til, 1895, p 2. 



