CEPHALOPODA. 343 



retain the sinus in the striae marking the ventral side. On the inner sur- 

 face the sinus is very sharp and abrupt. The exterior striae are bent in a 

 rounded, but distinct sinus, which is much broader than on the interior. 



The internal mould is essentially smooth, showing a slight, longitudinal 

 furrow on the ventrum over the siphuncle, and traces of the crenulations, as 

 continued over the walls of the air-chambers. 



The specimen described is nearly entire, preserving the chamber of habi- 

 tation and twenty-two air-chambers, and has a length of about 160 mm., with 

 a greatest lateral diameter of sixty mm. 



This species is distinguished by the curvature of the tube, the characters of 

 the chamber of habitation, and the surface ornaments. It somewhat resembles 

 G. Fischeri, but the curvature is much greater, and the point of greatest trans- 

 verse diameter different. In several of its characters it agrees with G. camma- 

 rus, but is distinguished by its smaller size, the position of the point of greatest 

 gibbosity, and less marked crenulations. This species shows a greater amount 

 of arcuation of the tube than is exhibited in any of the forms of this genus here 

 noticed. As preserved in the calcareous and pyritiferous septaria occurring in 

 the soft shales, the specimen retains its form and structural details in a very 

 satisfactory condition. It belongs to an assemblage of species in the Hamilton 

 group, characterized by their robust form and large size, and includes G. 

 Fischeri, G. solidum, G. abruptum, G. manes, G. poculum and G. lunatum. The 

 species following, belonging to this formation, are for the most part small, 

 slender and ovoid forms. 



Formation and locality. In the shales of the Hamilton group, at Hamburg, 

 Erie county, N. Y. 



