22 PAUEONTOLOOY OF NEW-YOBK. 



The forms 7 a, b, c, d, e, are varieties of the same species. The ventral valve of 7 d is 

 remarkably elevated, and the whole figure rotund. In 7 a, 6 & c, the form is more qua- 

 drangular, with a strongly marked sinus and elevated ribs upon the ventral valve. 



This species is exceedingly abundant at Chazy, forming almost of itself thin layers 

 of the limestone in which scarcely another fossil appears ; it is frequently crushed, and 

 presents a great variety of accidental forms. 



I have received specimens of the same fossil from Mr. Logan, Provincial Geologist of 

 Canada, marked (on the authority of Mr. Phillips) Terehratula tripartita ; but our shell 

 differs essentially in its proportions from that fossil, the length and breadth varying little, 

 and the radii never bifurcating, though the mesial elevation is broad, dividing the surface 

 *' into three nearly equal parts." 



Fig 7 a. View of the ventral valve, showing the beak of the dorsal valve closely incurved over the 



beak of the former. 

 Fig. 7 b. Lateral view of the same individual. 

 Fig. 7 c. Dorsal valve of a small specimen. 

 Fig. 7 d. Front view of a large specimen. 

 Fig. 7 t. Dorsal valve of the same. 



Position and locality. In the thin layers of impure limestone near the upper part of the 

 mass, and sometimes in the more compact portions. Chazy, Clinton county ; Galway, 

 Saratoga coimty ; and near Monlreal. {State Collection.) 



35. 4. ATRYPA PLICIFERA (n. 5p.). ' 



Pi.. IV. (bis). Figs. 8 a, b, c, d. 



Both valves depressed-convex, somewhat tetrahcdral, varying to broadly oval ; beak 

 small, acute, elevated and slightly incurved over the beak of the ventral valve, which is 

 closely appressed to the lower valve ; surface marked by about 18 or 20 equal rounded 

 radii, about 5 or 6 of which, on the dorsal valve, are slightly depressed, forming a shallow 

 sinus, and the same number very slightly elevated upon the ventral valve. 



The specimens 8 a, and 8 c, differ slightly in form, and the shell of one is more de- 

 iquamated than the other, but they are specifically identical. The number of radii in this 

 and the last described species are the same, but in this they are more rounded, the shell 

 altogether more depressed, and the sinus very shallow, scarcely extending more tlian half 

 the length of the shell. 



This species occurs with the last ; and when they are crushed and distorted, as most of 

 them are, it is not easy to separate individuals of the two species. 



Figs. 8 a, A. View of ventral valve, and front of same individual 

 Figs. 8 «, i. Ventral valve and front view of another individual 



Position and locality. In thinbedded shaly and siliceous limestone, near the upper part of 

 the Chazy limestone. Chazy, Clinton county. (State Collection.) 



