20 
PALEONTOLOGY OF NEW YORK. 
presenting on fracture or exfoliation tire character of the shell of a Nautilus 
or Baculites of the Secondary rocks. When the apex remains covered, it 
might be mistaken for a reversed shell; the depression on the upper side of 
the spire being sometimes deeper and more abrupt than on the lower side, 
as the plane of the first volution is below the centre of the shell, and the spire 
is shown only in the first, or first and second volutions. 
The specimens figured are essentially casts, preserving the surface only in 
a partial degree. The thickness and peculiar texture of the shell are unlike 
any of the other Gasteropoda in the same formation. 
This and the following species are placed at the end of the series of Platyce¬ 
ras, as indicating their doubtful relationship to that group of shells. 
Formation and locality. In the Upper Helderberg limestone, at Williamsville, 
Erie county, N. Y. 
Platyceras Ammon. 
PLATE VIII, FIGS. 7-10. 
Platyceras Ammon, Hall. Descriptions of New Species of Fossils, etc., p. 9. 1861. 
“ “ “ Fifteenth Rep. N. Y. State Cab. Nat. Hist., p. 37. 1862. 
“ “ “ Illustrations of Devonian Fossils: Gasteropoda, pi. 8. 1876. 
Shell depressed, suborbicular, making about two or three volutions, with the 
summit of the spire on the same plane or a little below the plane of the 
outer volution. Spire small; volutions contiguous throughout their whole 
extent, very gradually expanding above; the last half of the body-whorl 
ventricose. Aperture large, subovate, deeply sinuate on the left anterior 
margin. 
Surface marked by fine concentric undulating striae, which are deeply arcuate 
on the back of the last volution, corresponding to the sinuosity of the 
aperture; the striae are aggregated in lamellose folds or ridges, giving 
an irregular or undulating surface to the shell. 
This species has the form of Platyostoma ; but the peristome shows no col¬ 
umella, and presents a wide umbilicus. The length of the largest specimen, 
