CEPHALOPODA. 331 



The large aperture is elliptical, with the longer diameter nearly three times 

 the length of the transverse diameter. 



Air-chambers regular, numerous, having a depth of eight mm. There is 

 but a slight variation in the depth from the apex to near the grand chamber, 

 where several of the air-chambers are shallower than the preceding. Septa 

 smooth and thin, with a concavity greater than the depth of the air-chambers. 

 Sutures straight and horizontal. 



Siphuncle submarginal, moniliform, exposed in the process of weathering ; 

 expanding between the septa to a diameter equal to the depth of the cham- 

 bers, or twice its diameter at the septa. 



Test and surface-markings unknown. 



The internal mould of specimens, preserved in a coarse limestone, is essen- 

 tially smooth, without traces of the surface-markings or crenulations, but 

 showing the sutures considerably impressed from the process of weathering. 



A large individual, nearly entire, preserves the chamber of habitation with 

 about twenty-three air-chambers, and has a length of 220 mm. The greatest 

 transverse diameter of the specimen is 110 mm., and the aperture has a 

 width of eighty-four mm. 



This species is distinguished by its general form, the characters of the chamber 

 of habitation, and the aperture. It somewhat resembles G. impar in the features 

 of the grand chamber ; but that species is less gibbous, the septa much more 

 frequent, and the point of greatest transverse section is on the outer chamber. 

 In G. eximium the shell is more gibbous, the air-chambers deeper, and the 

 siphuncle larger and internal. 



Formation and locality. In the Upper Helderberg limestone, at Lexington, 

 Scott county, Ind. 



