906 NEW YORK STATE MUSEUM 



is characterized by a broad furrow on the ventral valve, and 

 a shallow furrow on the dorsal, a combination which causes a 

 notch in the margin. The beaks are broader and blunter than 

 those of the foregoing. The cardinal extremities are almost 

 angular. This is a very distinct type which seems to have been 

 derived from A. umbonata. 



Locality. Found only at Canandaigua lake. Total specimens 2. 



Genus tropidoleptus Hall 

 Tropidoleptus carinatus Conrad, mut. pygmaeus nov. 



Plate 3, fig. 12 



The outline of this form is truncated elongate oval. The 

 hinge line slopes a trifle to the acute cardinal extremities. A 

 typical shell measures 1% mm in breadth, 2 mm in length. 

 The surface is covered by 13 radiating plications, of which the 

 middle one does not reach more than halfway to the beak. 

 The ribs are scarcely as wide as the intervening furrows. This 

 mutation differs from the typical adult T. carinatus as 

 follows : the shell is higher than wide ; there are fewer plications ; 

 the ridges are not as wide as the furrows; the cardinal extremi- 

 ties are rounded; and the diameter is one fifteenth as great. In 

 all these respects the pyrite form in a striking manner resembles 

 a young T. carinatus. Its only variation from the young 

 phase is in the median ridge not reaching the beak. 



Locality. The specimens occur as ventral valves at Canan- 

 daigua lake. Total specimens 8. 



Genus strophalosia King 

 Strophalosia tmncata Hall, mut. pygmaea nov. 



Plate 2, fig. 10, 12 



Suboval shells, the hinge line sloping to either cardinal 

 extremity, the margin being a broad, even curve. The cardinal 

 extremities are angular. A typical specimen measures 2y 2 mm 

 in width and l 2 / 3 mm in hight. The ventral valve is strongly 

 convex, and shows a large scar of attachment on the umbo, in 

 the figured specimen % mm in diameter. The bases of spines are 

 scattered over the surface with no real regularity, but there is 

 an average of five in a concentric row. Compared with the 



