M PALiEONTOLOGY OP NEW-YORK. 



and the divaricator muscular impressions are broad, spreading, and some- 

 what truncate on their lower margins. The lateral and basal margins are 

 vsually a little more strongly marked by the impressions of the surface 

 •triaa ; and sometimes there is a partial or entire elevation of the part, 

 or a callosity more or less defined. 



Figures 6 & 7, Plate XI, arc of the natural size of the shells. 



Figures 9 A 10 of Plate xi are illustrations of an extreme form of the cast of this shell, 



where the median depression is much wider than usual, and the lateral margins show 



a partial callosity, 

 A oast taken from an impression of the dorsal valve, which occurs in the same association 



with the ventral valves, gives the characters shown in figure 8 of Plate xi, differing 



but slightly from figure 4 : this difference may be owing to the imperfect preservation 



of the parts in a coarse material. 



Among the collections are two or three specimens, which, preserving the shell 

 more or less completely, have the size and form of the casts. In one of these, where 

 the shell is apparently entire, the surface of the middle and lower part of the shell 

 is marked by distant elevated striaj with wide interspaces, which do not show 

 radiating striae, but are marked by concentric striae. On the umbo, the surface is 

 marked by wide radiating bands without distinct strisB. 



In another partially exfoliated specimen, the surface near the margin of the 

 shell shows minute striae between the coarser ones ; and the same characters are 

 partially shown in one or two other specimens. 



In the collections before me there are about thirty specimens with the characters 

 described, and all these are of nearly the same size ; nor can I trace any connexion 

 between these and any of the larger forms. Notwithstanding, therefore, that I am 

 opposed to creating new species where it can bo avoided, and knowing that the 

 species of this genus have been unnecessarily multiplied, I must regard this as a 

 distinct and well-marked form. In its dimensions, it corresponds with the next 

 described species, but differs in its surface markings and form of muscular im- 

 pressions. It is nearly of the same size as S. nacrea, but differs in the greater 

 convexity of the ventral umbo, and in the form of the muscular impressions as 

 ■well as in the surface-markings. 



The illustrations on Plate xv show the form of muscular impression, surface 

 ■trie, etc 



Geological formation and locality. In the Schoharie grit at Clarksville and Knox 

 in Albany county. The species is not at present known in the Corniferous limestone. 



