GASTEROPODA. 
15 
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 pinus 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 
fifteen 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 
