456 



LAMELLIBRANCHIA TA, 



[TRO. 



TBOCHOLITES, Conrad, 1838, Ann. Geo. 

 Rep. N. Y., p. 119. [Ety. trochoa, hoop ; 

 litfios, stone.] 

 Discoidal vo- 

 lutions in the 

 same plane, 

 about four, 

 rounded, 

 slightly con- 

 cave on the 

 ventral side, 

 gradually 

 e nlargi ng 

 toward the 

 aperture; 

 septa direct; 



Fi G .764.-Trocholites 

 ammonias. 



siph uncle 



ventral. Type T. ammonius. 

 ammonius, Conrad, 1838, Ann. Geo. Rep. 

 N. Y., p. 119, and Pal. N. Y., vol. 1, p. 

 192, Trenton, Utica, and Hud. Riv. Grs. 

 circularis, Miller & Dyer, 1878, Cont. to 

 Pal., No. 2, p. 9, Hud. Riv. Gr. 



minusculus, Miller & Dyer, 1878, Cont. to 

 Pal. No. 2, p. 9, Utica Slate Gr. 



FIG. 765. Trocholites circularis. 



planorbiformis, Conrad, 1842, Jour. Acad. 

 Nat. Sci. Phil., vol. 8, p. 274, and Pal. 

 N. Y., vol. 1, p. 310, Utica and Hud. 

 Riv. Grs. 



CLASS LAMELLIBRANCHIATA 



[Ety. lamella, a thin plate ; branchiee, gills.] 



THE Lamellibranchiata, Blainville, or Conchifera, Lamarck, have bivalve 

 shells, abound in the rivers of North America, in every ocean, and were common 

 in all geological ages, back to early Silurian time. The river shells are known by 

 the common name of mussels, and nearly all belong to three genera, Unio, Ano- 

 donta, and Margaritana. All known Palaeozoic shells of this class inhabited salt 

 water. The animals have a bilobed mantle, the sides of which secrete a calcareous 

 shell having two valves, which are attached by some kind of a hinge. The hinge 

 frequently has teeth on one valve that fit in cavities on the other. The valves 

 being on each side of the animal, one is a right valve and the other a left valve. 

 In most genera the valves are equal, and the animals lived in an erect position, 

 resting on the edge of the shell opposite to the hinge, and, when moving, plowed 

 a furrow in the sand or mud by the extension of a tongue-like foot. In some 

 genera one valve is much larger than the other, and the shell lies on the larger 

 valve, and adheres to some foreign object, as is the case with the common oyster; 

 in other instances the locomotion is by suddenly opening and closing the valves, 

 which causes the shell to dart through the water, first in one direction and then an- 

 other, as the Pecten does. Some genera have a byssus by which they are attached 

 to submarine bodies. Each valve commences to grow at the apex or beak, which 

 is also called the umbo. The umbones are almost always directed toward the ante- 



