16 
PALAEONTOLOGY OF NEW YORK. 
larger” and “more rapidly expanding,” and “having much more numerous 
spines.” For the purpose of comparison with the New York forms, I have 
introduced on plate 6, two figures of a very fine specimen of this species from 
the Upper Helderberg group, near Columbus, Ohio, kindly loaned to me 
by Dr. J. S. Newberry for this purpose. 
Platyceras dumosum var. rarispinum. 
PLATE V, FIGS. 5-7, 10. 
Platyceras dumosum var. rarispinum, Hall. Descriptions of New Species of Fossils, etc., p. 10. 1861. 
“ “ “ “ Fifteenth Rep. N. Y. State Cab. Nat. Hist., p. 38. 1862. 
“ “ “ “ Illustrations of Devonian Fossils: Gasteropoda, pi. 5. 1876. 
Shell consisting of about two or more volutions. Apex closely enrolled for 
more than one volution, and sometimes the volutions are contiguous nearly 
to the aperture ; for one volution and a half the apex is slender and gently 
enlarging; the body-volution expanding more rapidly below, and becoming 
moderately ventricose, depressed on the dorsum, and the left side some¬ 
times marked by a strong fold, indicating a sinus in the margin of the 
peristome, commencing at an early stage of growth. Aperture somewhat 
round, oval or slightly transverse. 
Surface marked by wrinkled concentric striae, which are strongly undulated at 
the base of the spines, and often abruptly bent backward at other 
places. Spines scattered; comparatively strong, from five to fifteen or 
seventeen on shells from the medium to the largest size. 
The larger shells have a length of less then two inches, with a height of one 
inch and a quarter; the greatest diameter is one inch and a quarter. 
This shell is never so ventricose as the P. dumosum proper, and the dorsum 
is often flattened while in that it is always convex. The P. dumosum is a com¬ 
paratively rare shell, while the var. rarispinum is not uncommon, and I have 
not seen any of the latter which have attained the size of the full-grown indi¬ 
viduals of the former. 
The P. dumosum cited by Mr. Conrad as occurring in the Hamilton group is 
