7« PALEONTOLOGY OP NEW-YORK. 



Theae Tariations arc illustrated on Plato ix, where figures 1 & 2 are typical forms of the 

 ■peoicfl, in some of which the area is inclined forwards. 



FiQS. 3, 4 & 5 are views of dorsal valves which are proportionally longer, but have a regular 



convexity. 

 Fias. 6 & 7 are of a larger dorsal valve of similar form, but more irregular exterior. 

 FlQB. 8 A 9 are the dorsal and ventral valves of a slightly distorted specimen. 

 Fia. 10 u a ventral valve which is slightly unsymmetrical. 



FiQ. 11 is a symmetrical ventral valve, which has the form of area and muscular impression 

 of S. pandora. 



Via. 12 is an enlarged figure from a cast in arenaceous shale. The strioB appear coarser than 

 in the softer shale. 



In a slab with numerous specimens, the apices of some are a little distorted, and others are 

 quite symmetrical. 



Geological formation and locality. This variety occurs in the Hamilton group, 

 at York, Moscow, Pavilion, Darien, Cauandaigua lake, and Bristol, Ontario county ; 

 and near Tully, Onondaga county. 



Var. G. Streptorhynchns perversa. 



PLATE IX. 



Orthi* perverta : Hall, Tenth Report on the State Cabinet, 1857, p. 97. 

 Orthisina altemata : Hall, Thirteenth Report on the State Cabinet, 1860. 



Shbll Bubelliptical, wider than long ; the cardinal extremities rounded, 

 and the hinge-line less than the width of the shell. Length and width 

 about as two to three ; the sides curving shortly, and the front broadly 

 rounded. The ventral valve is very convex at the umbo and the beak 

 distorted, somewhat depressed below the umbo, and nearly flat or un- 

 equally depressed-convex on the middle and lower part : area nearly 

 vertical, with the apex inclined or arcuate ; foramen closed by a pro- 

 minent convex pseudo - deltidium. Dorsal valve rounded, convex or 

 Bometimes gibbous; the greatest convexity above the middle, and 

 sloping abruptly to the hinge-line, sometimes flattened or a little de- 

 pressed in the middle towards the front. 



SoBFACB marked by distant elevated striae which increase mainly by 

 interstitial additions, and the intermediate spaces are crossed by ele- 

 vated concentric undulating striae. 

 In my description of this species, I remarked that it "is of the tj'pe of Orthif 



" umbraculum, and presents the irregularities of form common to that shell." 



