180 FIFTEENTH KEPORT OF THE CABINET OF NAT. HISTORY. 



rfr 



fibrous. Valves articulating by teeth and sockets. Interior of ventral valve with 

 two short diverging dental lamellae, which extend into an^ are affixed to the 

 sides or bottom of the rostral cavity. The muscular impressions occupy a narrow 

 triangular cavity below the bases of the lamellae, and usually extend about one- 

 third the length of the shell. Dorsal valve with a well-defined median septum, 

 which extends half the length of the shell : the hinge-plates are narrow and 

 strong processes, embraced by the curving teeth of the opposite valve. 

 In numerous specimens examined, there is no evidence of internal spires; and 

 it is only the similarity of these forms to Merista and Spirigera, that affords an 

 argument in favor of the existence of these appendages. 



The types of this genus are Atrypa quadricosta and j^. mesacostalis ( Hall, 

 Geol. Report of the Fourth District of New-York = Meristella quadricosta and 

 M. multicosta described in this paper, from the shales of the Hamilton group. 



The following figures will serve to illustrate more fully the characters and dif- 

 ferences of these genera, as given above. 



■9^M 



Fig. 1 & 2 are of the exterior, and .3, 4 the interior and cast of Athyris spiriferoides from the 

 Hamilton group. N.York. 



Fig. 5, 6 & 7 : Dorsal, front and profile views of Merista princeps of the Lower Helderberg. 

 8, 9 are the interior and cast of the ventral valve as shown in a species of the age of 

 the Upper Helderberg limestones, Ohio : the letter x refers to the filling of rostral 

 cavity ; t, the cavities of the dental lamella ; K, the triangular muscular impression. 



0-\A^-' l^'-"V<^Cr# f^-<_ t^ I 



V ^ 



7f 



