GASTEROPODA. 
13 
Platyceras rictum. 
PLATE IV, FIGS. 6, 12-17. 
Platyceras rictum, Hall. Descriptions of New Species of Fossils, etc., page 7. 1861. 
“ “ “ Fifteenth Rep. N. Y. State Cab. Nat. Hist., p. 35. 1862. 
“ “ “ Illustrations of Devonian Fossils: Gasteropoda, pi. 4. 1876. 
Shell very depressed, arcuate, or sometimes obliquely subcorneal, the width 
equalling and often exceeding the height. Nucleus minute, the spire en¬ 
rolled to about one turn and a half, when it abruptly expands, spreading 
more upon the right posterior side and becoming greatly extended and 
expanded in front. The dorsal line of the first volution is continued in 
an oblique angular ridge extending on the left side, and often reaching to 
the front of the shell giving a peculiar obliquity to the form. Aperture 
expanded, obliquely ovate; peristome entire, or with a slight sinuosity on 
the left posterior side, and sometimes showing the effect of the undefined 
folds of the posterior margin usually contiguous to the preceding volution. 
Surface marked by undulating concentric striae, and a few broad undulations 
longitudinally; width of aperture from one inch to one and three-fourths, 
and a little greater from the anterior to the posterior margin; height 
about one inch and varying from three-fourths of an inch to an inch and 
a quarter. 
Formations and localities. In the Upper Helderberg limestone of Onondaga 
county, and in the shales of the Hamilton group in Ontario county, N. Y. 
Platyceras echinatum. 
PLATE V. FIGS. 1-4. 
’ Platyceras echinatum, 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 small, obliquely subovoid. Apex closely incurved for about one volu¬ 
tion or one and a half; the body-whorl, from one-half to one volution, 
