FOSSILS OF THE NIAGARA GROUP. 131 



the united valves. . The specimen referred to measures about an inch 

 and a half in transverse diameter, by about one inch and three-eighths 

 from beak to base on the dorsal side. The projection of the beak on the 

 ventral side would make the length and breadth about equal. As the 

 specimen will be mostTy in the , condition of casts of the interior, they 

 will show the features of the interior of the shell ; or, in other words, 

 the impression of the interior surface. In the middle of each valve, 

 and extending from near the beak to the center, or beyond the center of 

 the shell, there is an elongate triangular depression, with two obtusely 

 pointed, triangular processes rising from the lower part of the depres- 

 sion, one on each side, and having their extremities directed toward the 

 beak. These processes are closely appressed to the surface of the cast 

 when preserved (but are frequently broken), and are very thin. The 

 surface outside of the depression is regularly convex, except near the 

 rostral portion, where it slopes abruptly to the level of the hinge plate, 

 and shows on the ventral side the rounded or convex triangular space 

 left by the depression of the triangular, foramen-like area beneath the 

 beak. The triangular depressions in the middle of the valve is formed 

 by the removal of the internal transverse muscular plates, or transverse 

 septum, which has been supported along the middle by a vertical parti- 

 tion, with cavities on each side beneath the plate, the two triangular 

 processes having filled these cavities. The form and proportion of this 

 plate has varied considerably among different individuals of the species, 

 as is seen by the depressions left in several specimens examined from 

 the typical localities. The discovery of this species, and a species of the 

 genus Monomerella (M. Neivherryi), at the same locality, is a matter of 

 considerable interest, when taken into consideration with the occurrence 

 of such numbers of Trimerella at other localities of the formation in 

 other parts of the State, giving reason to expect the development of all 

 these forms in some one locality if properly sought for. 



Gekus monomerella, Bill. 

 Monomerella Newberryi (a sp.). 



Plate 7, figs. 1, 2. 



Shell elongate orbicular, approaching oval in outline, with depressed 

 convex valves, giving a lenticular profile. Dorsal valve a very little 

 more ventricose than the opposite, and fullest on the umbonal region. 

 The muscular plate in the interior of the dorsal valve, judging from the 

 form of the internal cast, has been very large and broad, apparently 



