CEPHALOPODA. 291 



harie grit ; but the air-chambers have a greater depth, the chamber of habita- 

 tion is not so much developed, and the apical angle is larger. 



Formation and localities. Hamilton group, on the south shore of Lake Erie ; 

 at Cayuga lake ; and at Cazenovia, Madison county, N.Y. It has also been iden- 

 tified in the rocks of this formation at Cumberland, Maryland. 



Orthoceras Telamon, n. sp. 



PLATE LXXXV, FIGS. 3, 4, 11 



Shell straight, regularly and gradually enlarging from the apex. Transverse 

 section circular. Apical angle about 4°. Initial extremity unknown. 



Chamber of habitation cylindrical, well developed, having a length about 

 three times the diameter of the tube at the last septum, and without 

 any marked constriction or expansion toward the aperture. Air-chambers 

 regular, more frequent toward the grand chamber, having a depth of five mm. 

 where the diameter of the tube is nine mm., and of eight mm. where the 

 diameter is twenty mm. The ventral side is indicated in the cast, by a low 

 longitudinal carina along the walls of the air-chambers. 



Septa apparently smooth and thin, with the margins sometimes slightly 

 thickened; having a concavity equal to an arc of about 110°. Sutures 

 straight, and somewhat oblique in a dorso-ventral direction. 



Siphuncle large, excentric, less than one-third the diameter of the tube 

 from the ventral walls ; having a diameter of two mm. where the tube 

 measures twenty mm. in diameter, and diminishing but slightly toward the 

 apex. Its elements have not been observed in its passage through the air- 

 chambers. 



Test and surface-markings not preserved on any of the specimens exam- 

 amined. 



Internal mould smooth, with the exception of the ventral carina. 



This species is distinguished from O. exile by the ventral carina on the cast 

 of the interior, the comparatively deeper air-chambers, and the more excentric 

 position of the siphuncle. It closely approaches 0. emaceratum in several fea- 



