318 



Pelecypocla. — The classic memoir of Jackson (32) on the phylogeny 

 of the Pelecypocla brings together numerous illustrations of recapitula- 

 tion among the members of this class of animals. Jackson's conclusions 

 are well-known, and I shall therefore review them very briefly. 



From a study of a large number of genera representing widely diver- 

 gent members of the Pelecypoda, Jackson concludes that there is present 

 throughout the group an embryonic shell, which he calls the "prodisso- 

 conch" (a term correlative with the term protoconch of the Cephalopoda 

 and Gastropoda ),, and which is a simple bivalved, equivalve shell. At this 

 (phylembryoriic) stage of development there are two adductor muscles, 

 even in genera in which the adult have only one adductor. That is, the 

 prodissoconch is dimyarian even though the adult animal may be mono- 

 myarian. In the Aviculidre and their allies (Ostrea, Avicula, Perna, Pec- 

 tvn, Plicatula, Anomia) the prodissoconch very closely resembles in form 

 the primitive genus Nucula. The anatomical characters of the prodisso- 

 conch also bear out this resemblance. It is therefore inferred that some 

 such type as Nucula is the primitive ancestor of the Aviculida?, and pos- 

 sibly of the Pelecypoda. The paleontological and anatomical evidence 

 supports this conclusion. 



We have here, then, in the Aviculidre and their allies, a group of 

 monomyarians, some of them, as Ostrea, Plicatula, and Anomia, of very 

 aberrant form, the representation in the ontogeny of a dimyarian stage, 

 which, from all the evidence, actually characterized the adults of the 

 ancestors of the group. Whether or not Nucula is the actual ancestor of 

 this group of pelecypoda, it is quite certain that the earliest pelecypods 

 were of the same general form as the prodissoconch, and that they were 

 dimyarian. 



In the same paper Jackson has shown in a masterly manner that 

 the ostreaform shape of the shell, which characterizes many more or less 

 widely separated genera of pelecypods, is due to "the mechanical con- 

 ditions of direct cemented fixation." These ostreaform shells are very 

 variously derived, and should, if there is anything in the theory Of re- 

 capitulation, each show in the young stages, before the valves have be- 

 come fixed, the distinctive adult characters of its particular ancestor. In 

 this case we are relieved from the danger of arguing in a circle by the 

 fact that the genetic relations of most of the forms are fairly well known 

 from lines of evidence other than the ontogeny. The following specific 

 cases cited by Jackson are of especial interest. 



