60 



Silurian Fossils of Canada. 



equal but sometimes uniform in size, from ten to fifteen in the 

 width of two lines, crossed by excessively fine crowded concen- 

 tric lines. 



This species is allied to S.fluctuosa, but diff'ers in having the 

 areas of the ventral and dorsal valves inclined, at an angle which 

 is rather less than a right angle. In its outline it forms nearly an 

 uniform arch instead of being abruptly bent like S. fiactuom. 



The dorsal valve of S. recta (Conrad) is said to have a slight 

 mesial depression, while the ventral valve is flat. >S^. piano-con* 

 vexa (Hall) has also a slight mesial depression in the dorsal 

 valve, and is flat or even a little convex in front of the beak of 

 the ventral valve, where this species is concave. It has also a 

 perforated beak, and an area more approximated to the plane of 

 the lateral margins that it is in S. Thalia. The three species 

 are, however, notwithstanding these difl'erences, closely related. 



Locality and Formation. — Trenton limestone, City of Ottawa. 



Collector. — E. Billings. 



Strophomena Hecuba. N. s. 



Fig. Y. — Strophomena Hecuba^ dorsal valve. 



Description. — Subtriangular with usually a linguiform projec- 

 tion in front. Width on hinge-line about two inches ; length 

 varying from a little less to a little more than the width. 



Dorsal valve very convex, nearly regularly arched from beak 

 to front, only a small space at the hinge extremities compressed, 

 the whole of the remainder of the shell exceedingly ventricose, 

 usually a rounded fold in front which becomes obsolete at one 

 fourth the length, area sublinear, scarcely half a line in width in 

 the largest specimens. 



