121 



The ventral valve is in the small specimens, depressed convex and nearly 

 uniformly arched from beak to front ; the umbo well defined, but the concave 

 depressions on each side rather obscure ; no deflected margin. The large 

 specimens (nine lines wide) are sometimes strongly convex. Dorsal valve 

 concave, its curvature corresponding to that of the ventral valve. Surface 

 as in S. alternata. 



Area of ventral valve half a line in height in a specimen seven lines 

 wide, lying nearly in the plane of the margin, apparently a little sloping 

 outwards, forming an angle of about 100° with that of the dorsal valve, 

 which latter is scarcely one-fourth of a line wide. Foramen not distinctly 

 observed but apparently wider than high. 



The detached and empty ventral valves exhibit two rather large trian- 

 gular hinge teeth, one on each side of the foramen, covered with striae on 

 the outside in a manner similar to that of the area of those species to which 

 Professor Hall has given the generic name of Strophodonta. 



The spiniform ears are often either broken or worn away. 

 Varieties. — Several specimens nine lines wide without ears, and others 

 of the same size strongly convex, and with an indistinct deflected margin, 

 occupying from one-third to one-half the length of the shell, appear to 

 belong to this species. 



The species when the ears are broken away has exactly the appearance 

 of S. alternata, only that it is never more than half the size. The charac- 

 ters of the hinge areas and teeth taken together with the small size and 

 hinge ears, are abundantly sufficient to show that it is distinct from S. alter- 

 nata. It is a longer shell than S. nitens, and is in general destitute of a 

 deflected margin. It is smaller, thinner and less convex than S. Ceres. 



Locality and Formation. — Anticosti, in strata situated from 800 to 1000 

 feet above the base of the Middle Silurian, and 250 feet below the rocks 

 containing Pentamerus ollongus. 



Collector. — J. Richardson. 



