76 TlfENTON PEUIOD. 



dorsal valve have, and become laterally compressed near the beak; plica- 

 tions about seven on each side of the sinus, becoming smaller toAvard, 

 and finally obsolete at, the postero-lateral regions. Both valves have the 

 ph cations distinctly defined to the beaks, and, being sharply angular at the 

 front, they give the margin there a sharply zigzag appearance. Besides 

 the plications, the surface of both valves is marked by regular and distinct 

 lines of growth, which appear more prominent upon the plications than 

 between them, giving them a delicately-knotted or crenulated appearance 

 in some shells. 



Length and breadth, each about ten millimeters; height, about eight 

 millimeters. 



Separated valves of this species somewhat resemble those of B. siib- 

 trigonalis Hall, and also, to a less extent, B. Anticostiensis Billings. From 

 the first, it differs in its more compact form and its more elevated mesial 

 fold; from the latter it differs, in similar particulars, and also in its greater 

 approach to an angular outline. 



Position and locality. — Lower Silurian strata of the Cincinnati epoch ; 

 Silver City and Upper Mimbres Mining Camp, New Mexico. 



Class CONCHIFERA. 



Order HETEROMYARIA. 

 Family MYTILID^. 



Genus MODIOLOPSIS Ball, 1847. 



Modiolopsis (?). 



Among the collections made at Upper Mimbres Mining Camp, New 

 Mexico, are some specimens of a Conchifer that evidently belong to the 

 genus Modiolopsis Hall. They are too imperfect for specific characteriza- 

 tion, and are noticed only for their generic value in stratigraphical determi- 

 nations, and for the piu-pose of giving a full account of the fossils collected 

 at that interesting locality. 



