4«» PALJEONTOLOGY OF NEW- YORK. 



The dorsal valve shows strong hinge-plates which are sometimes much thickened 

 and very prominent, preserving a minute foramen lielow the apex, which may be 

 closed or communicate with the cavity of the shell below. The crura are strong and 

 round at their bases, becoming abruptly slender, sometimes pi'oceeding directly and 

 sometimes slightly diverging, giving oflF in one direction the slender points which 

 penetrate the cavity of the ventral valve, and in the other direction bending into 

 the cavity of the dorsal valve, and at the same time spreading laterally. The crural 

 plate is variable in form from broad to very narrow lanceolate, nearly straight or 

 more concave on the upper or rostral side, with a slender process issuing from the 

 centre and directed obliquely upwards into the cavity of the ventral valve. The 

 muscular impression is double and strongly marked in well-preserved specimens; 

 the median septum more or less strongly defined, but in no instance has it been 

 observed to rise so high as the bottom of the crural plate. 



Fig. 1. Dorsal view of a epecinien of a symmetrically oval form. 



Fig. 2. A specimen showing a mere prominent bealc and contracted front. 



Fig. 3 a, b. Ventral and dorsal views of a specimen of less extreme form than the preceding. 



Fig. 4. An elongate form which is more symmetrically oval. 



Fig. 5 a. Dorsal view of a small specimen of the prevailing or characteristic form. 



Fig. 5 b. ProBle view of the preceding, showing a scarcely perceptible incurving of the valves 

 at their lateral margins. 



Fig. 5 e. Dorsal view of an older specimen, showing the perfect condition of the shell. 



Fig. 5 d. Profile view of the same, showing the inflected margins of the shell. 



Fig. 5 «,y & g. Dorsal, ventral and profile views of a full-grown specimen of this species. 



Fig. 6. A cast of a similar form. 



Fig. 7. The interior of two ventral valves, one preserving the deltidial plates and the other 

 without them. The muscular impressions are but feebly preserved. 



Fig. 8. The interior of two dorsal valves ; one showing the hinge-plates in their usual form, 

 and the upper one having them much thickened : in the latter, the crura diverge 

 at their origin, while in the other they proceed in a longitudinal direction for a 

 short distance before bifurcating. 



Fig. 9. The dorsal valve, showing the hinge-plates, dental fosscts, foramen, crura and crural 



plate, and appendages. The figure is slightly enlarged. 

 Fig. 10. The figure of a similar specimen, where the hinge-plates arc much thiclconcd and 



rounded, the crura branching near their base, and the crural plate narrower than 



in the preceding. 



Fig. 11. A diagram showing a longitudinal section of the two valves in connexion, with the 

 crura and appendages. 



Fig. 12, 13, 14 & l."). Figures showing the variety of form of the crural plate, as observed 

 in several individuals of this species. 



Geological position and locality. In the Oriskany sandstone : Cumberland, Md. 



