58 TRANSACTIONS OF THE WAGNER FREE 



many forms. It may be supposed that in the prototypic bivalve it had a flat, 

 crawling surface, such as is found in Nucitla, Yoldia, and other recent but 

 archaic forms. It is possible that in the Nucnlidic, Ledidie, Arcidce and Soi- 

 emyidic the existing groove, which, by expansion, can be converted into a 

 flat surface, is a survival. A somewhat similar surface is produced by an ex- 

 pansion of the foot in some species of Macoma, Trigonia, Lepton, Callocardia 

 and Galcoinma, and here can hardly be anything but a special modification of 

 the ordinary type, adapted to the peculiar habits of the species, since there is 

 no permanent groove. The modification of such a surface into the hood-like 

 form which the foot assumes in certain deep-water Pectens would be easy, and 

 in those cases it has the aspect of a sucker. 



In other groups we find a phalliform ( Verticordia, Poromya), clavate (Ln- 

 cina), or digitiform (Mytilus) organ. In the Solenidte and Mycetopiis the foot 

 is obviously modified for boring purposes, and, in the Pholads, as a triturating 

 organ. We conclude, therefore, that the form of the foot, like that of the 

 heart, is of little service as a basis for systematic classification, except in minor 

 subdivisions. 



In a few cases, such as Ostrea, it is altogether aborted, though remnants 

 of its retractor muscles exist and are attached to the valves. 



In a few mollusks, such as Plioladoinya and Halicardia, an accessory fin- 

 like organ, or opisthopodium, is developed at the posterior end of the visceral 

 mass. It is possible the small, valve-like aperture noted in Pa no pea, Lnlraria, 

 and many Anatinacea, may be a reminiscence of a time when the opisthopo- 

 dium was more common and required a foramen in the adherent mantle edge 

 for the performance of its functions. 



Faint traces of the grooving on the ventral edge of the foot occur in many 

 Pelecypods. The deeply-grooved foot, which can be expanded so that the 

 sides of the groove when in one plane will form a surface upon which the ani- 

 mal may crawl or with which it can push, may in the interest of brevity be 

 called reptary ; the lingrooved foot, which can be expanded as a plane, subrcp- 

 tary. 



The ProdissOCOnch. The embryonic shell, corresponding to the proto- 

 conch of Gastropods, has been named by Jackson the Prodissoconch. In 

 general these valves are very uniform in character, as seen on the tips of the 

 uneroded valves of the adult. They are usually rounded or slightly pointed 

 at the umbonal end, in their earliest stages having sometimes a straight, rather 

 long hinge-line. In Solcmya the prodissoconch is elongate, rounded at the 

 ends, with the ventral and dorsil margins nearly parallel, much as in the adult 

 shell. In Pinna the prodissoconch is globular as in most bivalves. In Unio 

 and Anodon a second or nepionic stage is traceable, owing to a semiparasitic 

 habit of the young, which leave the mother and become encysted on the fins 

 or gills of fishes ; during this period the shell remains stationary, though some 

 development of the contained soft parts is in progress. This nepionic stage 



