PHYLOGENY OF THE PELECYPODA. 355 



special conditions, as the cementation of a valve, or by anatomical features and habits of 

 the individual. I think therefore that the peculiarity of the shell in Anomia may be con- 

 sidered as an adaptation to its environment' and not due to any genetic connection of the 

 group. On this question we have the support of the conclusions of Professor Lacaze- 

 Duthiers, who in his well-known paper on Anomia ejjhippium, L., says: "... 

 I'ossicule de I'Anomie est un byssus, et toutes les anomalies sont la consequence de la 

 position de I'animal sur la cote droit, et de la soudure du byssus aux corps etrangers." 

 His studies were anatomical, based on the ambiguous adult and it is my pleasure as a 

 result of studies of the young, to be able to show conclusively the correctness of some 

 of the deductions of the eminent French investigator. 



The youngest specimens found alive on glass slides were about the size of PL xxix, 

 figs. 5-6. At this age and up to a considerably later period of growth, the young are 

 freely locomotive, crawling actively at intervals by prolongation of the highly developed 

 foot. The lines of gi-owth in fig. 6 indicate that, when very young, the byssus was on 

 the extreme margin of the valves; but in this figiu-e the byssal sinus has already become 

 sub-central in position and paitially enclosed by the growth of the right valve. At this 

 age the byssal fixation is only maintained a part of the time, or perhaps more correctly 

 speaking the byssal attachment may be said to be voluntarily and frequently abandoned. 

 The byssus of this early period is composed of a series of bundles of threads which flatten 

 against the surface of the glass as seen from the right side through the glass to which it 

 is attached. The byssal hold at this age is an act of volition on the part of the animal, 

 as if paralyzed with cocaine they maj be easily washed ofi" a glass slide by a stream from 

 a pipette, whereas in the normal condition they cannot be dislodged by any force short 

 of scraping. The cocaine treatment is a good method to employ in collecting very young 

 Anomiae attached to shells, rocks, etc., because if scraped off, the lower valve, which, in 

 this species, is of extreme tenuity, is almost certain to be destroyed. When older the 

 byssal attachment becomes permanent by the calcification of its tip. 



A young Anomia viewed fi-om the left side is shown in PI. xxix, fig. 1, and the reverse 

 or right side of the same individual seen through the glass to which attached is shoAvn 

 in fig. 2. The relations of these views to one other and to the adult described by La- 

 caze-Duthiers are of much interest. The single adductor muscle of the valves, ad, 

 figs. 1-2 (also in the diagram PI. xxix, fig. 3), is situated a short distance below the 

 heart, h. It is very small relatively to the size normally found in Pelecypods, a fact hav- 

 ing important bearings. This is the only muscle indicated in the left valve which passes 

 to the right valve or which finds its counterpart in that valve and the same condition ex- 

 ists in the adult. A large byssal muscle, bm, fig. 1, inserted in the left valve passes to 

 the foot and byssus, the calcified attachment of which is shown at bs, fig. 2. The byssal 

 muscle at its insertion in the left valve has a peculiar sinuosity in its posterior face 

 which is not understood. A posterior retractor of the foot, pr, figs. 1 and 3, is attached 

 to the left valve a short distance from the byssal muscle and close to, but separated from, 

 the adductor. It is somewhat remarkable that the posterior reti-actor lies on the ventral 

 border of the adductor, as the typical position of the posterior retractor in Pelecypods 



'In Anomia, the slicU itself is not cemented to foreign is l)y means of the calcification of the byssal tip, and is 



bodies as in the ostrean forms considered ; the attachment not to be confounded with the cementation of one valve. 



