General Classification of the Pelecypoda. 103 



Bernard's Classification. 



Bernard's discovery of the discontinuity of the embryonic 

 and adult dentitions of the Taxodonta, and his working out 

 of the development of the definitive teeth, overthrew the 

 hypothesis that the crenulations observable in certain Hete- 

 rodonta postulated the descent of the latter from the former. 

 On the other hand, lie regarded the Heterodonta as being 

 derived from an early taxodont {i. e. dysodont) ancestry by 

 tiie specialization of the lateral lamellae — that is to say, he 

 considered these lamellae to be homologous in both groups, 

 for he says : — 



"Pour comparer la charniere des Heterodontes a celle des 

 Taxodontes, il sera necessaire de s'adresser, non pas aux 

 formes adultes mais aux formes embryoniques . . , Une dent 

 des Taxodontes sera homologue non pas a I'une quelconque 

 des dents Heterodontes adultes mais a Pune de leurslamelles 

 primitives qui se recourbent . . . pourra donner uaissance, 

 suivant les cas a 1, 2, ou 3 dents definitives/' 



In his work Bernard clearly states that the Taxodonta 

 and Anisomyaria form one group, the latter showing clearly 

 the evolution of taxodont teeth from internal ribs. This is 

 also shown by the Areas in the former group. The Aniso- 

 myaria show the beginning of the taxodont dentition, but 

 not its full development. The Mouomyarian dentition he 

 shows to be degenerate — indeed, Ostrea never passes through 

 a taxodont stage. The absence of a well-developed taxodont 

 stage may of course be due either to want of phylogenetic 

 development or to a similar degeneration. Into this point 

 Bernard does not go. The evidence of the Monorayaria and 

 the specialized habit of the Anisomyaria generally point to 

 its non-development being due to degeneration. That this 

 loss of later specialization threw more and more work on 

 to earlier stages is shown by Ostrea, which, never passing 

 through a taxodont stage, has embryonic crenulations 

 persisting late. 



As the Taxodonta and Anisomyaria are included in one 

 order, that order cannot very well be called Taxodonta. A 

 name which seems suitable is Pleurodouta, as it refers to the 

 definite proof of the evolution of the taxodont teeth from 

 internal ribs. As to the names of the two suborders, Taxo- 

 donta is ])erfectly suitable ; but the name Anisomyaria cannot 

 very well stand, as it seems to show an order in a general 

 classification based on the considerations of the hinge, 

 divided off because of its muscular characters. For the 



