306 PAUBONTOLOGF OF NEW YORK. 



This species resembles 0. Thyestes, with which it is associated, and is distin- 

 guiahed from that species by its more gradually enlarging tube, cylindrical 

 chamber of habitation, and much deeper air-chambers. In the size of the shell 

 and distance between the septa this species belongs to a group of large forms, 

 including 0. Thyestes, 0. Eriense, 0. Zeus and O. Pelops ; but the position of the 

 siphuncle and the concavity of the septa is very different. 



Formation and localities. In the calcareous layers of the Portage group at 

 Penn Yan, Yates county, and in the shales of the group at Rogers' Bridge, on 

 the Genesee River, below Portageville, N. Y. 



Orthoceras Thyestes, n. sp. 



PLATE LXXXVI1I, FIG. 2. 



SHELL large, straight, robust, regularly and rapidly enlarging to the chamber of 

 habitation. Transverse section not observed on account of the compressed 

 condition of the specimen described. Apical angle about 11°. Initial 

 extremity unknown. 



Chamber of habitation not satisfactorily observed. A portion having a 

 length of 100 mm. shows a gradual contraction toward the aperture, which 

 in an entire individual may prove to be merely a constriction of the tube, at 

 a distance from the anterior termination. Air-chambers regular, increasing 

 in frequency from the outer chamber to the apex, varying in depth from 

 . twelve to five mm. on a portion of the tube having a length of 220 mm. 



Septa and siphuncle not observed. Sutures straight and horizontal. Test 

 and surface-markings unknown. 



Internal mould essentially smooth, with the sutures slightly impressed. 



The specimen figured, consisting of a portion of the chamber of habitation 

 and twenty-four air-chambers, has a length of 310 mm. 



This species is associated with O. Atreus, and distinguished by its rapidly 

 enlarging tube, and much shallower air-chambers. 



Formation and locality. From the soft shales of the Portage group near Wat- 

 kins, at the southern end of Seneca Lake. 



