GASTEROPODA. 117 



being that of interrupted longitudinal striae, with the intermediate spaces 

 reaching the even surface of the shell. 



Formation and localities. This species has thus far proved a rare form, though 

 having rather a wide geographical distribution in the shales of the Hamilton 

 group. It occurs near Summit, in Schoharie county ; at Sherburne creek, in 

 Chenango county ; at Borodino, on Otisco lake ; at Norton's Landing, on the 

 east side of Cayuga lake ; and at York, in Livingston county. With one excep- 

 tion, only a single specimen has been obtained at each of these localities. 



sfkviks of the chemung group. 

 Bellerophon triliratus. 



I'l.ATK xxiv, figs, s, ie-w. 



Bdlerophtm (Phraffmnftomal) tricarinata. Hall. Illustrations* of Devonian Fossils : Gasteropoda, pi. 22. 1876. 

 Not B. tricarhiatus. Shimard. 1858. 



Shell subhemispheric. Inner volutions comparatively small, the outer one 

 extremely dilated, and in its normal condition very ventricose. Aperture 

 greatly expanded, the peristome flattened and slightly repand, with a 

 rather strong and moderately deep sinus in the anterior margin ; auricu- » 

 lated posteriorly and nearly enclosing the volution in that direction. 



Surface marked by distinct revolving striae, which in the casts are indicated 

 by sharp narrow grooves, with a flattened interspace of three or four times 

 the width, the direction being often slightly undulating. Finer and less 

 conspicuous stria? mark the entire surface and become more distinct towards 

 the margin. The dorsal line is marked by a bicarinate band, which when 

 well-preserved, shows a narrow depression of about one-sixteenth of an 

 inch wide at the margin, and half that width at the columella, bounded 

 on each side by a sharply defined line. On each side of this carina and 

 equidistant from it, is an angular fold extending from the columella, from 

 one-third to one-half the length of the expanded portion of the volution, 

 and gradually becoming obsolete. The expansion is sometimes marked by 

 three or four concentric wrinkles, having a regularity which might seem 



