Fig. 12. Cardinal view of an internal cast; showing the filling of the spon- 
dylium and of the visceral foramen, and the cavities left by the 
median septum and hinge-plate. 
Fig. 13. Dorsal view of an individual of medium size with regularly elliptical 
form and finely striated surface. 
Corniferous limestone. Western N. Y. 
Fig. 14. Median longitudinal section of both valves; showing the degree of 
development of the spondylium, median septum, and septal plates; 
also the great thickness of the shell in the umbonal region of the 
brachial valve and the length of the crura, with their convex 
terminal expansion. : 
Corniferous limestone. Le Roy, N. Y. 
Fig. 15. The exterior of a brachial valve having the characteristic subovate 
outline. 
Corniferous limestone. Cayuga, Ontario. 
257 
