62 FIFTEENTH REPORT ON THE CABINET OF NAT. HISTORY. 



Surface marked by distinct undulated revolving strise, which be- 

 come less conspicuous towards the aperture : obscure lamellose 

 striae of growth mark the surface transversely. 



In this species the sides of the volution are less convex, and the dorsum 

 more distinctly angular (or subcarinate) than in the preceding species. The 

 equal undulating revolving striae constitute a conspicuous difference between 

 the two. In the casts, faint impressions of the revolving striae are preserved. 



Geological formation and locality. In the coarse shales of the Hamilton 

 group at Hamilton and at Pratt's falls in Madison county, N.Y. 



GENUS THECA (Sowerby). 



THECA LIGEA (n.s.). 



Shell elongate, triangular, compressed, very gradually tapering to 

 an acute extremity. Length about twice the width, and the dia- 

 meter from side to side equal to half the width. Margins thin and 

 sharp; one side flattened or slightly convex, with a depressed line 

 on each side a little within and parallel to the margin : opposite 

 side convex, angular in the middle. Section triangular; the peri- 

 stome on the flattened side produced beyond the opposite, curved 

 and apparently thickened at the margin. 



Surface marked by transverse lines of growth, which have been 

 arched upon the angular side. 



The specimen is a cast in limestone, preserving faint indications of the 

 transverse striae. 



Geological formation and locality. In the Upper Helderberg limestone 

 at Clarence-hollow, Erie county, N.Y. 



GENUS CONULARIA (Miller). 



CONULARIA UNDULATA. 



Conularia undulata : Conrad, Ann. Rep. Palaeontology of New-York, 1841, p. 57. 



" Distinguished from C. quadrisulcata by having the strise more 

 " crowded and undulated, and by the absence of lines crossing 

 " the furrow between the strise." 



This fossil is not rare in some localities of the Hamilton group. The 

 longitudinal striae crossing the furrows are obsolete, or nearly so, on the 

 upper and middle portion of the shell, and sometimes faintly perceptible near 

 the base. 



[ September, 



