KINE TO GENESIS. 265 



found in many widely separated genera of pelecypods, 

 as Ostrea, Mulleria, and Tridacna ; thus proving the 

 development of the same features on different lines of 

 descent.^ In Aspergillum the two valves have con- 

 cresced so as to form a truly univalvular, tubular shell, 

 so that the adductors would evidently be functionless if 

 existent. The posterior adductor has disappeared and 

 the anterior is reduced to a few disconnected shreds 

 (Fisher), though evidently existent in the young, as 

 attested by the form of the shell in the nepionic stage. 



"Ordinarily there are two posterior retractor- mus- 

 cles of the foot in pelecypods, one situated on either side. 

 In adult Pecten either the left retractor alone exists, 

 or both retractors are wanting (the left doubtless al- 

 ways exists in the young). In studies of young Pecten 

 irradians, I found that the animal always crawled 

 while lying on the right side, with the foot extended 

 through the notch in the lower valve and pressed 

 against the surface of support. It is evident that while 

 crawling in this position the left retractor is in the 

 plane of traction, and it is retained ; on the other hand, 

 the right retractor would not be in the plane of trac- 

 tion, and it has disappeared through disuse and atro- 

 phy.'-^ A similar disappearance of the right retractors 

 of the foot is seen in Anoi/iia glabra, and is explained 

 on similar bases of argument. 



"In My a arenaria we find a highly elongated siphon. 

 In the young the siphon hardly extends beyond the 

 borders of the valves, and then the animal lives at or 



IDr. B. Sharp and I published almost simultaneously closely similar 

 views on the mechanical aspect of the relative size of the adductors. See 

 Proceeds. Fhila. Acad., 1888, p. 122, and Proceeds. Boston Soc. Nat. Hist., Vol. 

 XXIII., 1888, p. 538. 



2 Both retractors doubtless exist in the prodissoconch stage of Pecten and 

 allies. 



