GASTEROPODA. l5 



more upon the right side, which is sometimes depressed-convex, while the 

 left side is more abruptly rounded. The aperture is subrhomboid-ovate, 

 with the peristome making a sinus on the left side, the posterior margin 

 widely separated from the preceding volution. 

 Surface marked by strong concentric striae, which are interrupted and irreg- 

 ular from the numerous nodes projecting from the shell, and extended 

 into long tubular spines. 



Formation and localities. From the Upper Helderberg group, Helderberg 

 mountains, in Albany and Schoharie counties, N. Y. ; Columbus, Ohio, and 

 Falls of the Ohio river. 



This shell, in its full size, attains a length of two and a half inches, with the 

 entire surface covered with strong spines which are sometimes two inches in 

 length. In well-marked specimens, these spines are arranged in diagonal 

 rows across the body of the shell in a nearly regular quincunx order. One 

 specimen shows the bases of ninety of these spines, and, from the extent 

 of the shell, the whole number must have been more than one hundred. 



Mr. Conrad's description is as follows : "Shell covered with thick tubular 

 spines, arranged in longitudinal rows ; margin of aperture waved ; volutions 

 free." He remarks that, " in some varieties the spines are comparatively 

 few." 



In some specimens of about one-half the full size, there are not more than 

 fifteev or twenty spines ; and in all those with few spines, the expansion of 

 the body-whorl is much less abrupt, and they rarely attain more than half 

 the size of the large typical forms of the species. The number of volutions in 

 the smaller forms is fully equal to the larger ones, being two or more, with 

 the last one quite free. 



Since this form is so distinct and constant, I propose to designate it as 

 variety rarispinum. 



The Platyceras multispinosum of Meek, figured in the Geological Report 

 of Ohio, presents the form and proportions of well preserved specimens 

 of P. dumosum as they occur in the limestone of New York, but is some- 

 what larger than any specimens we have seen. It is described as " much 



