126 



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

 depression, while the ventral valve is flat. S. plano-convexa (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 than it is in S. Thalia. The three 

 species are, however, notwithstandnig these differences, closely related. 



Locality and Formation. City of Ottawa, Trenton limestone. 



Collector. E. Billings. 



STROPHOMENA HECUBA. N. s. 



STROPHOMENA HECUBA, Can. Nat. and Geol., Vol. V. p. 60, Feb. 1860. 



Fig. 104. 

 Fig. 104. Strophomena Hecuba, dorsal valve. 



Description. Subtriangular with usually a linguiform projection 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. 



Ventral valve depressed convex near the beak, and concave all round 

 near the margin, area about one line wide and forming an angle of about 

 115 with the plane of the lateral margins. Foramen not observed. 



Surface marked with fine radiating striae ten or twelve in the width of 

 one line, every third, fourth or fifth one of which is twice the size of the 

 intermediate fine ones. The whole surface is besides (in most specimens) 



