20 PAL.EOSTOLOGY OF NEW YORK. 



presenting on fracture or exfoliation the character of the shell of a Nautilus 

 or Baculites of the Secondary rocks. When the apex remains covered, it 

 might he mistaken for a reversed shell; the depression on the upper side of 

 the spire heing sometimes deeper and more abrupt than on the lower side, 

 as tin' 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, 



