119 



The beak, umbo depressions on each side of the umbo and cardinal 

 angles of the ventral valve the same as in jS. alternata. The deflected 

 margin forms an angle of between 100 and 110 with the general plane 

 of the visceral disc, and occupies on the median line (in all the specimens 

 I have seen) from one third to nearly one half the whole length of the 

 shell. 



The dorsal valve is quite flat, or even a little concave, just in front of 

 the beak, but elsewhere curved to correspond with the ventral valve. 



The area of the ventral valve lies nearly in the plane of the lateral 

 margins, and the area of the dorsal valve forms with it an angle of about 

 95. The height of the area of the ventral valve at the foramen is three 

 fourths of a line in a specimen nine lines wide, and of the dorsal valve 

 about one third of a line. Foramen of ventral valve partly closed by a 

 V-shaped deltidium, the lower open part of which is closed by the strongly 

 projecting deltidium of the dorsal valve. 



The width of the foramen, is about equal to its height. 



The surface is the same as in S. alternata^ and, when a little worn pre- 

 sents a smooth shining silken lustre. 



When compared with S. incrassata, 8. alternata, 8. deltoidea, S. cam- 

 erata, or 8. tenuistriata, it will be seen that this species is shorter in 

 proportion to the width than any of them, and also that the inclination of 

 the areas towards each other differs in forming an obtuse instead of an 

 acute angle. 



Locality and Formation. This species occurs at Charleton Point, 

 Anticosti, in the upper part of the Hudson River group. 



Collector. J. Richardson. 



STROPHOMENA CERES. (Billings.) 



STROPHOMENA CERES. Canadian Naturalist and Geologist, vol. 5, p. 54, Feb. 1860. 



Description. Semi-oval, sides rather straight and a little converging 

 for one third their length ; front angles and margins broadly rounded. 

 Width on hinge line twelve to fifteen lines ; length ten to twelve lines. 



The ventral valve varies greatly in the amount of its convexity. In 

 some specimens it is depressed convex, and these have almost precisely 

 the aspect of the more flattened forms of S. alternata. Others are slightly 

 convex, nearly hemispherical, uniformly arched from beak to front, no 

 devected margin distinct from the visceral disc, the latter occupying the 

 whole of the shell, except a small triangular space at the hinge-angles. 

 Between these two extremes there are individuals which present all the 



