MEDINAN, NIAGARAN, AND CHESTER FOSSILS 
55 
less vertical direction, while a corresponding elevation occurred 
immediately in front of the beak of the left valve. The elevations 
may be regarded as incipient teeth. (Plate XIV, figs. 7 A, B.) 
The fauna described by Hall from strata 50 to 60 feet below 
the top of the sandstone at Medina includes Lingula cuneata, 
Modiolopsis orthonota, Modiolopsis primigenia, Pleurotomaria 
pervetusta and Bucanica trilohita. This appears to be a Grimsby 
sandstone fauna. Modiolopsis primiqenia is represented by fig. 
8 on plate XIV. 
6. Ctenodonta (?) sp. 
Plate XIII, fig. 5 
A single left valve, 6.6 mm. long, 4 mm. high, and 1.3 mm. 
deep, resembling Modiolopsis orthonota in outline, but without 
any trace of concave curvature along a distinctly defined post- 
umbonal slope. In fact, there is no distinctly defined post- 
umbonal slope, the valve rounding from the general convexity 
of its middle parts into a more increased convexity along the 
posterior cardinal margin of the shell without any interruption 
whatever. In specimens of Modiolopsis orthonota of the same 
size the post-umbonal slope is distinctly defined and is distinctly 
concave. 
7. Ctenodonta (?) creditensis Sp. nov. 
Plate XIII, fig. 8 
Three valves in which the ratio of the height to the length 
varies from 71 to 76 per cent are at hand. In the two larger 
specimens the posterior part of the outline is defective. One 
of the specimens (fig. 3) is 8.5 mm. in height, and its length is 
estimated at 11.8 mm. In the other one of the larger specimens 
the height is 7.5 mm., and the length is estimated at 10.5 mm. 
The third specimen is 4.2 mm. in height and 55. mm. in length. 
The larger specimens are 2 mm. in depth. The anterior parts of 
these valves are similar to those of Modiolopsis orthonota, but 
the beak is more pointed and is directed more distinctly toward 
the front, and the upper half of the anterior outline, anterior to 
