ORISKANY SANDSTONE. 457 



This species varies greatly in form; some individuals being very gibbous and 

 distinctly ovate, while others are more or less compressed and very elongato-oval or 

 narrow elliptical : the first variety was Eaton's Terebratula oroides, and the latter 

 his T. perovalis. In the young state, the length and breadth are often equal or nearly 

 equal. 



After stiidying a large collection of these forms, I am satisfied that' they are va- 

 rieties of the same species, dependent on age and the conditions surrounding thera 

 during their existence. 



PLATE CIV. 



Fig. 1 a. Tlie dorsal side of a young specimen, where the length little exceeds the width ; 

 the shell nearly all exfoliated. 



Fig. 1 b, c. Ventral and profile view of the cast of a larger individual, preserving a little 

 of the shell. 



Fig. 1 d. Cardinal view of a cast of a specimen of medium size, which is more ventricose 

 than 1 6, c. 



Fig. 2 a, b. Dorsal and profile of a large specimen of elliptical form, preserving a part of 

 the shell in front. This is much less gibbous than usual in specimens of the samo 

 size. 



Fig. 3 a. Cardinal view of a large cast, the ventral valve uppermost. 



Fig. 3 6. Dorsal view of an individual of ovate form, preserving the greater part of the shell. 

 Fig. 3 e. Cardinal view of a gibbons specimen. 



Fig. 3 d. Cardinal view of an extremely gibbons specimen, which is flat or slightly concave 

 at the sides, and the centre of the ventral valve strongly elevated. 



Fig. 4. A fragment of stone presenting the aspect of these fossils in their usual condition in 

 the rock. 



PLATE CV. 



Fig. t. Dorsal valve of a full-grown individual, showing the abrupt inflection at the sides. 



Fig. 2. Profile of a large shell in which the parts are pretty well preserved in all their 

 proportions ; the dorsal valve being partially exfoliated, and the striae not visible. 



Fig. 3. Ventral view of the preceding. 



Fig. 4. Dorsal view of a large specimen, which is unusually contracted at the sides on the 

 lower half of the shell. 



Fig. 5. A partial cast of a ventral valve, showing the form of the muscular impressions and 

 rostral cavity. The forms of the dental plates and teeth are seen on each side, 

 below the beak. 



Fig. 6. A partial cast of a dorsal valve, showing a process reaching from the beak to the 

 mu.scular impressions below. This process is the filling of the foramen, and ex- 

 tends from the cavity of the dorsal valve beneath the bases of the crural supports, 

 coming out at the apex of the beak. 



Geological -position and localities. In the Oriskany sandstone : Albany, Greene, 

 Ulster, Schoharie, Otsego, Herkimer, Cayuga counties, and at nearly all localities 

 of the Oriskany sandstone in New-York; in Canada East and West, and in Penn- 

 sylvania, Maryland and Virginia. 



[ PALiKONTOLOaY III.] 58 



