riIYLOGP:NY OF THE PELECYl'ODA. 377 



conditions of the case.^ In support of tliis view, the bivalvular crustaceans may again be 

 cited as they have an analogous adductor muscle, developed, of coui'se, on an entirely 

 different line of descent, but under closely similar mechanical conditions. In the com- 

 pleted prodissoconch of all Pelecypods there are two adductor muscles, PI. xxiv, figs. 

 1-2, PI. XXX, fig. 4,^ thus arriving at what may be called in this respect the typical con- 

 dition of the class. Duiing later life the anterior, the posterior, or both adductors, may 

 be retained, reduced or lost, according as the persistence or changes in correlated feat- 

 ures of anatomy retain in use or bring into disuse the muscles in question. 



The preceding studies show (he existence of an embiyonic shell to which I have given 

 the name of prodissoconch (p. 311) in many widely separated genera of Pelecypoda. 

 We will now consider this early shell and see if any close connection can be traced be- 

 tween it and ancestral forms of Avhich it is the representative in early stages of develop- 

 ment. 



Two classes of prodissoconchs and succeeding shell structure ai-e described in the 

 preceding pages. In the fii-st division the prodissoconch has umbos directed posteriorly 

 and is not prismatic, but is succeeded by a dissoconch having an external layer of pi'is- 

 matic cellular tissue which is moi-e or less developed, but exists at least in the early 

 nepionic stages of one valve. The^e features are characteristic of the Aviculidae, Os- 

 treada?, PectinidiB and Anomiada% as shown by representative genera. The second di- 

 vision is less well defined; but with one exception can be separated from the fii-st. The 

 prodissoconch of this second division has umbos which are directed more or less ante- 

 riorly. It has no prismatic cellular layer and. is succeeded by a dissoconch which has no 

 prismatic layer. Such features are charactei'istic of genera repi'csentingthe families My- 

 tilidit, Arcidse, Chamida3,Petricolidjc, Cycladidse, Venerid0e,Glycinierida?, Scrobicularidas 

 and Myidae. The young of the Unionidfe as described (page 3b5) does not come 

 under this second section but in some features approaches the first. Only a part of the 

 families of Pelecypods are included under these two divisions. Whether other families 

 will come under one or the other, or yet new divisions, must be pj'oven by future inves- 

 tigations (see note, p. 375). 



The first of the divisions above considered embraces a group of families (the Avicu- 

 lida3 and allies) intimately bound together as I have attempted to show in this paper. 

 It will probably prove to be a group of ordinal value and I shall limit myself to this 

 grouj) in considering the derivation of the j^rodissoconch. The jii'odissoconch of the sec- 

 ond division has doubtless a diiferent ancestral kinship or perhaps several lines of ances- 

 tral kinship as the case may be. 



The features by which a possible connection may be traced to the i-adical ances- 

 tor from which the prodissoconch form is inherited iiicludes features of shell structure 

 and anatomy, for we know both in a measure from the results of the studies in this pa- 



'The mantle of molluscs often has a highly developed 'Also figures by Lovfiii, Sohuiidt and other investigators 

 muscular system, and it seems probable that the adductors of marine and fresh-water Peli'Cypoda The fact of the 

 of the I'elecypod arose from a modification of mantle two adductors is also attested by the form of the prodis- 

 muscles. A somewhat similar modification of these mus- soconch in the genera figured on I'lates xxvi-xxx inclu- 

 des is seen in the retractor muscles of the siphon, charac- sivc. 

 teristic of many Pelecypods, as the clam, PI. xxv, fig 0. 



