332 PJLjEOSTOLO&V OF NEW YORK. 



GOMPHOCERAS IMPAR, n. Sp. 



l'l.ATK -ll-l'I.KMKNT. 



Shell straight, exogastric. Transverse section broadly oval. Point of greatest 

 transverse section near the base of the chamber of habitation. Tube regu- 

 larly enlarging to the last septum, where it is slightly constricted. Apical 

 angle about 30°. 



Chamber of habitation large, with a length equal to the greatest ventro- 

 dorsal diameter. The dorsal side slopes somewhat more abruptly toward 

 the aperture than the ventral side. The tube gradually enlarges back of the 

 aperture, and suddenly expands to the point of the greatest transverse sec- 

 tion. The crenulated band in the cast forms a deep, narrow fossa at the base 

 of the grand chamber, and is crossed by furrows, of which there are about 

 four in the space of twelve mm. 



Aperture sub-oval, having a length equal to the greatest ventro-dorsal diam- 

 eter, or about twice its transverse diameter. The small aperture is indicated 

 by a sinus in the margins of the large aperture. 



Air-chambers numerous, regularly increasing in depth from the apex to 

 near the base of the outer chamber, varying in depth from four to six mm. 

 in the length of seventy mm. The last two air-chambers are shallower than 

 the preceding. Septa not observed. Sutures straight and horizontal. 



Siphuncle, test and surface-markings unknown. 



The specimen described has a length of 135 mm., of which sixty mm. per- 

 tain to the chamber of habitation, and preserves nearly seventeen air-cham- 

 bers ; being incomplete at the apex. 



This species is distinguished by its apical angle, the characters of the grand 

 chamber, and the frequency of the air-chambers. In the form of the chamber 

 of habitation it resembles G. mitra, but the point of greatest transverse section 

 is different, and the air-chambers much shallower. 



Formation and locality. In the Upper Helderberg limestone, at Columbus, 0. 



