196 PALEONTOLOGY OF NEW-YORK. 
255. 6 CYRTOCERAS ARCUATUM (n. sp.). 
Pu. XLII. Figs. 5 a, b, c. 
Broadly curving, and very gradually tapering ; septa approximate, thin ; section 
elliptical ; siphuncle dorsal ? 
This species has the appearance of a curved OrrHoceras; but I have seen several 
fragments of the same, all of which are curved, while I do not know a straight species 
having the same characters. The specimen figured is compressed, giving the section a very 
eccentric elliptical form. The siphuncle is obscure, but there is some indication of its 
existence upon the dorsal margin. 
Fig. 5 a. Lateral view of the specimen, which is crushed towards the lower extremity. 
Fie. 5 6, Section of the same. 
Fig. 5 c. The base of a parasitic coral, or of a crinoid, attached to the fossil. 
Position and locality. In the lower shaly layers of the Trenton limestone at Middleville. 
256. 7. CYRTOCERAS CAMURUM (x. sp.). 
Pu. XLII. Fig. 6. 
This species is somewhat similar in form to the last, but curving more rapidly. The 
septa are more distant, being once and a half those of the last species. The siphon is dorsal, 
as shown in some fragments of the same. 
This species is not rare in the lower strata of the Trenton limestone ; but it is almost 
always so intermingled with other fossils, that it cannot be separated except in fragments. 
It is clearly distinct from the other species, though it is at present impossible to designate 
the important characters of an entire specimen. 
Position and locality. In the lower strata of this rock at Middleville, in a situation where 
all the other species of the genus known in the Trenton limestone are found, two only 
being known in a higher position in the same rock. 
Genus ONCOCERAS. 
[ Greek, oyxos, a bending or protuberance, and xepas, a horn; the central portion of the fossil 
resembling a flexed and enlarged joint.] 
Character. Tube curved ; aperture constricted; lower part of the outer chamber, and 
upper part of the septate portion, ventricose, abruptly contracting towards the apex ; 
siphuncle small, dorsal ; septa plane, nearly flat, slightly elevated on the dorsal margin. 
This genus will probably be found restricted to the lower paleozoic strata. 
