CEPHALOPODA. 375 



The chamber of habitation has a length equal to once and a half the 

 diameter of the tube at the last septum. Crenulations preserved on the 

 internal mould, as low, rounded, longitudinal ridges, with fine intermediate 

 striae. Aperture straight and horizontal, entire, with a distinct sinus in the 

 ventral margin. 



Air-chambers regular, numerous, having a depth of 4.5 mm. Septa 

 smooth, thin, with a concavity greater than the depth of one air-chamber, or 

 equal to an arc of 110°. Sutures straight, and at right angles to the spiral 

 axis. 



Siphuncle near the ventral side, not expanding in the cavities of the 

 air-chambers. 



The test has a thickness of .75 mm. Surface marked by very fine undu- 

 lating, lamellose lines of growth, crossed by revolving striae and obscure 

 furrows. Tube ornamented with numerous transverse, plicate folds, or 

 expansions of the test, having an elevation of about two mm. on the outer 

 volution, and usually corresponding in frequency to the septa. Sinus distinct, 

 having a width of more than twice the depth. 



From the strength and regularity of the transverse folds of the test, the 

 internal mould is marked by annulations, which are more prominent on the 

 ventrum. It also shows lamellose lines of growth, and the furrows and striae 

 of the crenulations. 



The largest individual observed has a diameter across the disc of 190 mm., 

 with a diameter to the tube of forty-two mm. at the aperture. 



This species is distinguished from G. Nereus by the greater number of volu- 

 tions and the character of the surface ornamentation. In G. citum and G. 

 Matheri the transverse section is different, and the transverse plications are 

 more elevated and distant. 



The individuals of this species are comparatively common, and, as obtained 

 from the limestones, the casts are unusually well preserved. The examination 

 of a large number of specimens shows slight variations in the amount of 

 curvature of the tube, and in the prominence and frequency of the transverse 

 plications of the test. 



