CEPHALOPODA. 257 



He ascribes this ornamentation to markings on the surface of the mantle at the 

 base of the chamber of habitation, which have been impressed upon the suc- 

 cessive septa. In the species described in the present volume there is an evident 

 connection, functional or structural, between the ventral carina, the areola 

 around the siphuncle, and various markings and ornamentation of the septa, 

 and the ability of the mollusk to form an organic deposit in the posterior 

 air-chambers. 



Formation and localities. In the Schoharie grit at Schoharie, and Clarksville, 

 Albany county, N. Y. 



Section c. — Species of undetermined relations. 

 Oethoceras pervicax, n. sp. 



PLATE LXXIX, FIGS. 9-10. 



Shell straight, somewhat rapidly enlarging. Transverse section circular. 

 Apical angle about 10°. Initial extremity unknown. 



A portion of the chamber of habitation has been observed, possessing no 

 distinctive features. Air-chambers regularly increasing in depth from the 

 apex of the specimen to the grand chamber, varying from three to five mm. 

 The interior cast of the walls is smooth and flat, with the exception of a 

 distinct, raised longitudinal line or carina, extending the entire length of the 

 septate portion on the ventral side. 



Septa smooth, thin ; concavity equal to an arc of 90°. Sutures very 

 slightly oblique. 



Siphuncle small, having a diameter of two mm. where the shell has a 

 diameter of eighteen mm. ; excentric, nearer to the ventral side, the dis- 

 tance being about one-third the diameter of the tube. Its characters have 

 not been observed in the interseptate spaces. 



Test and surface-markings unknown. 



A specimen preserving a portion of the chamber of habitation, and twenty- 

 two adjacent air-chambers, has a length of 113 mm.; the diameter of the 

 33 



