PALEONTOLOGY OF NEW-YORK. 



area, which is broader above, and sometimes very much resembles this 

 feature in S. striatus. 



The casts preserve strong and prominent marks of the muscular im- 

 pressions, and sometimes vascular markings outside of the muscular 

 area ; while in other individuals, the papillose ovarian markings are 

 very distinctly preserved. In the cast of the dorsal valve there arc some- 

 times strong impressions of the occlusor muscular markings, and the 

 apex also shows the strias of the cardinal muscular attachment. 



It does not appear, from Mr. Conrad's description, that he recognized 

 the dichotomous ribs; but this feature is often obscure or scarcely recog- 

 nizable in the specimens from the Corniferous limestone in New York, 

 owing to partial exfoliation or other causes ; and in most of the smaller 

 specimens it does not exist, the division usually taking place beloW 

 the middle of the length. In full grown individuals, when the surface is 

 pretty well preserved, the duplicate character of the plications is a con- 

 spicuous feature. Somewhat rarely we observe a partial division of one 

 or more of the plications on different parts of the shell towards the 

 margin. 



A Spirifer in the Hamilton group, which I have identified as the 

 Delthyris prora of Conrad, is clearly identical with this one of the Corni- 

 ferous limestone. The dichotomous character of the ribs, however, is 

 more conspicuous, and the division often begins above the middle of the 

 shell (See fig. 24, pi. 34), and becomes a conspicuous feature in specimens 

 of medium size ; while in those of similar dimensions from the Cornife- 

 rous limestone, this character is scarcely perceptible. 



Tliis species is readily distinguished from all the other Si'ikifeka in our strata, 

 by the extremely elevated angular mesial fold and deep sinus, which are bounded 

 by the dichotomous plications. In an imperfect condition, it might be sometimes 

 mistaken for S. maa-othyris, which, in its hinge-area and the upper part of the 

 mesial fold, sometimes simulates the S. acuminalua. 



The young of this species are but moderately convex, \vith a broad depression 

 in front, as shown in lig. 9, pi. 29. From tlxis condition, there are uU degrees of 

 variation to the more ventricose and extreme forms as shown in figs. 12, 13 and 

 14, of Plate 29. 



