168 [Senate 



Orthis subumbona (n.s.). 



Shell somewhat plano-convex : dorsal valve nearly flat ; cardinal 

 extremities rounded : ventral valve very convex, gibbous, 

 length and height nearly equal ; hinge-line scarcely equal the 

 greatest width of the shell; beak little extended above the 

 hinge-line, abruptly incurved. Surface concentrically striated. 

 This shell resembles the Orthis p?mi/>ibo?ia; but is much smaller, less 



elongated, the length and width being nearly equal, and the beak much 



shorter and more abruptly acute. 



Geological position and locality. In calcareous layers in the shales of 



the Hamilton group : Shores of Seneca lake and of Lake Erie. 



Atrypa aspera. 



Atrypa ( Terehratula) aspera, Schlotiieim : Petrefactenkunde, pa. 263, pi. 



xviii, f. 3. 

 Atrypa spinosa, Hall : Geol. Rep. 4th Dist. 1843. 



There is propably no specific difference between the European species 

 and those from the Hamilton group of New-York. Specimens from the 

 shales of the Hamilton group in Iowa present some variation in form and 

 characters from those of New- York, while thoy approach more nearly the 

 European specimens in my collection. 



Atrypa hirsuta (n. s.)- 



Shell somewhat depressed orbicular in the young state, becoming 

 trilobate and more gibbous in older specimens ; sinus and 

 mesial elevation strongly marked towards the front; valves 

 nearly equally convex ; beak of ventral valve perforated. Sur- 

 face marked by from thirty-six to forty simple well defined 

 radiating strise or plications, which are crossed by fine concen- 

 tric striae and a few distant imbricating lamellae of growth. 



* Entire surface covered by extremely fine hair-like spines, 

 Avhich, on removal, leave a finely granulated surface. 

 The shells are usually denuded of the spines, and present the general 



appearance of young individuals of A. reticularis ; but the plications are 



