GASTEROPODA. 17 
a small variety of more ventricose form than the one here described, and 
has but few spines. 
Formation and localities. In limestone of the Upper Helderberg group, Helder- 
berg mountains, and in Oneida, Onondaga and Erie counties, N. Y.; Ontario, 
Ohio, and at the Falls of the Ohio river. 
A specimen from the Corniferous limestone of Columbus, Ohio, which I have 
illustrated on plate 5, fig. 13 of this volume, I have considered to be the young of 
P. dumosum, since it shows no characters by which it may be separated from 
specimens of the same size in the limestone of New York. The specimen on 
the same plate, fig. 11, might perhaps be referred to the variety attenuatum of 
Meek. But it seems scarcely possible to make reliable specific distinctions 
upon the greater or less number of spines or even upon the form of the shell in 
a genus subject to such extreme variations. 
Platyceras nodosum. 
PLATE VII, FIGS. 4, 5. 
Platyceras nodosus, Conrad. Fifth Ann. Rep. Pal. State of N. Y., p. 56. 1841. 
“ nodosum (Conr.), Hall. Geolog. Surv. N. Y.—Pal., vol. 8, p. 473, plates 115 and 116. 1859. 
Platyceras dumosum and P. dumosum var. rarispinum, Hall. Illustrations of Devonian Fossils: Gaster¬ 
opoda, plates 5 and 7. 1876. 
This form has occurred in two specimens only among all the collections 
made in the progress of the work. 
They are casts of the interior, preserving a stronger and more angular 
character than the ordinary forms of P. rarispinum, and are in all respects 
similar to the P. nodosum of the Oriskany sandstone, to which they are referred. 
Formation and localities. In the limestone of the Upper Helderberg group at 
Schoharie and the Helderberg mountains, N. Y. 
Platyceras undatum. 
PLATE VII, FIGS. 1, 2. 
Platyceras undatum, Hall. Illustrations of Devonian Fossils: Gasteropoda, pi. 7. 1876. 
Compare Platyceras subnodosum, Hall. Geolog. Surv. N. Y.—Pal., vol. 3, p. 474, pi. 115. 1859 
Shell elongated, arcuate. Apex incurved, nucleus unknown, the succeeding 
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