PALAEONTOLOGY 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 are some¬ 
times strong impressions of the occlusor muscular markings, and the 
apex also shows the striae 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 pr or a 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. 
This species is readily distinguished from all the other Spirifera 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 8. macrothyris , which, in its hinge-area and the upper part of the 
mesial fold, sometimes simulates the 8. acuminatus. 
The young of this species are but moderately convex, with a broad depression 
in front, as shown in fig. 9, pi. 29. From this condition, there are all degrees of 
variation to the more ventricose and extreme forms as shown in figs. 12, 13 and 
14, of Plate 29. 
