GASTEROPODA. 
11 
the P. ventricosum of the Lower Helderberg group; but the first volution is 
more slender, and the spire less closely enrolled, while the lateral pouch-like 
expansion and the revolving bands are distinctive features. This shell is also 
much smaller, being rarely more than an inch and a quarter high, with the 
greatest diameter of the aperture about equal to the height. The length 
from the apex to the anterior margin of the aperture is about one inch 
and a quarter in large individuals. The small specimen (fig. 7) is a very 
symmetrical form of a young individual of the species which preserves the 
peristome in contact with the preceding volution. The figure scarcely repre¬ 
sents the true character of the specimen. 
Formation and localities. In shales of the Hamilton group, at York and Moscow, 
in Livingston county, N. Y. 
Platyceras fornicatum. 
PLATE IV, FIGS. 1-5, 7, 8, 18, 19, 20, AND PLATE V, FIGS. 8, 9 [?]. 
Platyceras fornicatum, Hall. Descriptions of New Species of Fossils, etc., p. 7. 1861. 
“ ‘‘ “ Fifteenth Rep. State Cab. Nat. Hist., p. 35. 1862. 
“ “ “ Illustrations of Devonian Fossils, pi. 4. 1876. 
(?) “ “ v. contractum, Hall. Id., pi. 5, figs. 8, 9. 1876. 
“ rictum (part), Hall. Id., pi. 4, figs. 18, 19, 20. 1876. 
Shell obliquely subhemispherical, or very depressed, obliquely subconical. 
Apex minute, distinct, spirally enrolled for about one turn and a half, 
below which it expands, so that in the extent of an inch and a half along 
the dorsum to the front, it has acquired an aperture of about an inch and 
a half in diameter in both directions. The upper side of the spire for 
the first volution and the following half is flattened; the angle continu¬ 
ing into the broad expansion of the body-wliorl, and dying out before 
reaching the margin, as shown in figs. 1-5. Aperture nearly round or round- 
ovate ; peristome scarcely sinuous, except at the postero-lateral margin. 
Surface marked by fine concentric strife, with a few strong spines upon the 
body-volution. 
