CONTRIBUTIONS TO PALiEONTOLOGY. 65 



aperture round. Septa distant about one-third the diameter of the 

 tube. Shejl thin or free from nodes or ridges, except towards the 

 aperture, which appears to be thickened. Greatest diameter of the 

 shell across the volutions, about two inches : diameter of the 

 outer volution three-fourths of an inch. 

 Surface markings undetermined. 



This species is more slender in its volutions than the T. clio, with a much 

 wider umbilicus and a less elevated spire. Although I have seen but a single 

 specimen, the form and proportions clearly indicate it to be a very distinct 

 species. 



Geological formation arid locality. In the Schoharie grit : Schoharie. 



GENUS GYROCERAS (Meyer). 



GYROCERAS TRIVOLVIS (Conrad, sp.). 

 Cyrtoceras trivolvis : Conrad, Ann, Rep. Palssontology of New-York, 1840, p. 206. 



" Shell large, rounded, with transverse lines of growth : septa 

 " numerous." 



This species is the most common one known in the Upper Helderberg 

 limestones. So far as I know, however, it rarely or never reaches three 

 volutions. 



The tube is rounded ; volutions disjoined, gradually enlarging to the 

 aperture, which is not conspicuously expanded bejond the proportions of 

 other parts of the shell ; section of the volutions nearly circular, or round- 

 oval : septa distant about one-sixth of the diameter. A fragment three 

 inches long, measured along the centre of a longitudinal section, embraces 

 sixteen chambers, and has a diameter in the middle of one inch and a quar- 

 ter : the convexity of the septa, from the ventral to the dorsal side, is a 

 little more than twice the space between the septa. Siphuncle subcentral, 

 being a little excentric on the dorsal side. The shell is marked by close 

 lamellose transverse striae, and is raised in transverse ridges which are 

 distant sometimes the space of one and sometimes of two septa, and usually 

 directly over this part of the shell : these ridges have a slight retral bend 

 on the back of the shell, giving a similar sinuosity to the aperture. 



In many specimens the shell is not distinctly or decidedly annulate, but 

 presents transverse undulations which are less conspicuous on the ventral 

 side. 



The C. tricolvis is the most common species in the Upper Helderberg 

 limestones, and frequently measures six inches across the volutions. 



Geological formation and locality. In limestone of the Upper Helder- 

 berg group in the Helderberg mountains, Schoharie, Oneida and Onondaga 

 counties. 

 1861.] 9 [ Senate No. 116.] 



