108 PALEONTOLOGY OF NEW-YORK. 



appear to bo subject to such extreme variations in respect to the striae. It resem- 

 bles very closely, in its surface markings, the S. textilis, but is always smaller, with 

 a more extended hinge-line, and never so flat as that species. In the same respects 

 it is not very dissimilar to some of the other species of the Upper Heldcrborg and 

 Hamilton groups ; and it has been confounded with S. inequiradiata, ■which is a 

 much larger shell, ^vith different internal characters. Its surface strisB are more 

 delicate than any of the other species, and its muscular impressions are peculiar 

 in form and details, and entirely unlike those of any other species, as shown in 

 figures 2 A, i, k, on Plate xviii. 



The general characters of the species and varieties of form are shown in figures 

 2 a-k, on Plate xviii. 



After critical comparison of all the other species of the Hamilton group, I am 

 convinced that the species of Mr. Conrad is the one I have here described and 

 illustrated, although not entirely corresponding with the original description. 



Geological formations and localities. This species occurs in the Corniferous lime- 

 stone somewhat rarely, but is common in the Hamilton group, and is at present 

 unknown in any higher position. It is common in localities along the shores of 

 Seneca, Cayuga and Canandaigua lakes ; at Moscow ; York ; Pavilion ; Darien ; 

 Eighteen-mile creek, and numerous other places in the Hamilton group. Its locali- 

 ties in the Corniferous limestone have already been enumerated on page 88. 



Stropliodonta textilis. 



PLATE XVIII. 

 Strophomena (Strophodonta) textilit : Hail, 5n Tenth Report on State Cabinet, p. 141. 1847. 



Shell very thin, nearly flat, transverse, semioval, sometimes nearly as 



long as wide, the length being from two-thirds to four-fifths the width ; 



hinge-line equalling or sometimes a little greater than the width of 



the shell below, with salient angles. 

 Venteal valve slightly convex. Dorsal valve gently concave. 

 Abea of the ventral valve narrow, almost linear, and extremely narrow 



towards the extremities, vertically striated, with a narrow callosity in 



the centre ; the inner margin crenulate for about two-thirds its length. 



Area of the dorsal valve linear, scarcely half as wide in the middle aa 



that of the opposite valve. 



