280 F. B. Meek—Descriptions of new Silurian Fossils. 
give the means of making out its generic and specific characters 
with some degree of detail, though we yet want specimens show- 
ing the exact form of the entire shell, and the muscular impres- 
sions of the ventral valve. 
dium sometimes covering the upper part of the foramen. __ 
It is probably more nearly related to Stricklandinia of Bill- 
ings; but on comparing some of the specimens sent to him 
Prof. Orton, Mr. Billings writes that he thinksitentirely distinct 
from his genus, which he says has no such cardinal process. 
The ventral valve of our shell also differs in having no trace 
of the triangular internal chamber, seen under the beak of 
Stricklandinia. 
Dicraniscus Orronr M. 
Shell truncato-suboval, or suborbicular, with front narrowly 
rounded; hinge-line less than the greatest breadth. ioe 
moe 
erately deep mesial sinus not extending to the beak; cardinal 
ent, or at least more so than that of the other valve, and more 
incurved ; area wanting or very narrow and obscure; cardinal 
er, 
urrowed on their posterior sides above. Surface smooth. 
as the dorsal; because I could not understand how so very 
