94 



VAL^ONTOI.OOY OF NEW-YORK. 



CEPHALOPODA OF THE CLINTON (iKOUP. 



The fossils of this family are so rare as scarcely to attract attention, though there are a 

 few interesting forms which are almost entirely restricted to the lower part of the group. The 

 most remarkable genus is the Ormoceras, of which a single species occurs in great numbers in 

 the Black-river limestone ; while it is extremely rare in the Trenton limestone, and appears 

 again in the Hudson-river group, and there only as a single species. Until within a short period, 

 this genus has been unknown above the Lower Silurian period in our country ; but, from 

 fragments occurring in the strata under consideration, and in a higher position, it is probable 

 that this genus extends throughout the silurlan strata. 



489. 3. ONCOCER AS SUERECTUM (;!.*/?.). 



PL. XXVIII. Fig. 11 a, b. 



Fragment subfusiform or conical, abruptly enlarging from the smaller extremity, and 

 contracted towards the aperture, slightly curved ; surface marked by fine, slightly uneven, 

 transverse stria;. 



This specimen, at first view, appears like Gomphoceras ; but it still retains a slight curve, 

 increasing towards the smaller extremity, but which, from the specimen having been com- 

 pressed, is not so obvious in the figure. The transverse section is oval, but owing to pres- 

 sure towards the aperture that portion is quite flattened, so that the contracted aperture of 

 0^•cocERAS is not characteristic in the specimen. The surface is plainly striated, except on the 

 inner side of the curve for a short distance there is a suture, producing an abrupt arching in 

 the striae. 



Fig. 11a. Lateral view of the fragment, 



Fig. 11 i. A portion of the surface enlarged, showing a suture on one side. 



Position and locality. In the lower cherly layers of the group at Lockport. 

 490. 4. ORMOCERAS VERTEBRATUM. 



Pl. XXIX. Fig. 1 a - g. 



Shell elongated, gradually tapering, section circular ; siphuncle nearly central ; surface 

 unknown ; septa four to five in the space of an inch ; edges of the septa slightly projecting 

 in the cast, and the spaces between them contracted or concave ; siphuncle presenting the 

 usual appearance of Ormoceras, the longitudinal diameter of the expanded portions being about 

 one half as great as the transverse diameter. 



This species, which occurs in casts or fragments, bears considerable resemblance to Ormo- 

 ceras tenuifdum of the Black-river limestone ; but the septa are more convex and somewhat 



