66 FIFTEENTH REPORT ON THE CABINET OF NAT. HISTORY. 



GYROCERAS MATHERI ( Conrad, sp.). 



Cyrtoceras matheri : Ann. Report on the Palaeontology of New-York, 1840, p. 206. 



" Resembles the last, but the transverse ridges are more prominent 

 " and distant : they meet at an angle on the middle of the back." 



This shell makes from one to one and a half volutions. Shell somewhat 

 rapidly enlarging from the apex. Section nearly circular towards the apex, 

 and becoming transversely elliptical towards the aperture, so that the two 

 diameters are about as nine to twelve. Septa distant : where the dorso- 

 ventral diameter of the shell is one inch, the distance of the septa on the 

 side of the shell is nearly half an inch ; on the ventral side, three-eighths 

 of an inch ; and on the dorsal side, five-eighths of an inch. 



Surface marked by close lamellose striae, which are abruptly arched back- 

 wards upon the dorsum, and the shell raised in strong ridges corresponding 

 to the septa, which are abruptly bent backwards on the dorsal line. The 

 casts show elevations along the line of the septa, corresponding to the exte- 

 rior ridges, and the surface between is distinctly depressed. Faint revolving 

 bands mark the surface of the casts, and each band is composed of several 

 slender striae. Specimens of the ordinary size measure from four to five 

 inches across the greatest diameter. 



This species differs conspicuously from C. trivolvis in the less circular 

 form, more rapid enlargement, strong transverse ridges, and more distant 

 septa. 



Geological formation and locality. In the Upper Helderberg limestones 

 at Schoharie and near Catskill. 



GYROCERAS UNDULATUM ( Vanuxem, sp.). 



Cyrtoceras undulatus : Yanuxem, Geological Report Third District New-York, 1843, 



pp.139 &140, f. 2. 



Shell consisting of two or three free volutions, which are gradually 

 expanded towards the aperture. Volutions scarcely symmetrical; 

 section somewhat obliquely oval as in Trochoceras, though the 

 volutions are apparently in the same plane : transverse diameter 

 of the volution greater than the dorso-ventral diameter. The 

 septa are distant about one-fourth the ventral diameter. 



Surface strongly striate or lamellose, the lamellae arching back- 

 wards : on the dorsal line, and on the dorso-lateral angles, the 

 lamellae are sharply bent backwards at every fourth volution, 

 forming two rows of lamellose nodes or short spine-like processes. 

 The greatest diameter across the volution is about five and a 

 half inches. 



[ September, 



