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Pelecypoda.—The classic memoir of Jackson (32) on the phylogeny 
of the Pelecypoda 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 
(phylembryonic) 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 Aviculidze and their allies (Ostrea, Avicula, Perna, Pec 
ten, Plicatula, Anomia) the prodissoconch very closely resembles in form 
the primitive genus Nucula. The anatomical characters of the prodisso- 
concb also bear out this resemblance. It is therefore inferred that some 
such type as Nucula is the primitive ancestor of the Aviculidze, and pos- 
sibly ef the Pelecypoda. The paleontological and anatomical evidence 
supports this conclusion. 
We have here, then, in the Aviculidze 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 
frou: lines of evidence other than the ontogeny. The following specific 
cases cited by Jackson are of especial interest. 
