196 PALjEONTOLOOY of NEW-YORK. 



255. 6. CYRTOCERAS ARCUATUM ( n. sp.) . 



Pl. XLII. Figs. 5 a. b, e. 



Broadly curving, and very gradually tapering ; septa approximate, thin ; section 

 elliptical ; siphuncle dorsal 1 



This species has the appearance of a curved Orthoceras ; 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. 



Fig. 5 b. 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 Middle vi lie. 



256. 7. CYRTOCERAS CAMURUM ( 71. ip.). 



Pl. XLU. 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. 



f Greek, oyxos, a bending or protuberance, and xspas, 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 ; 

 sipliuncle small, dorsal ; septa plane, nearly flat, slightly elevated on the dorsal margin. 



This genus will probably be found restricted to the lower paIa;ozoic strata. 



