CONTRIBUTIONS TO PALAEONTOLOGY. 37 



PLATYCERAS AMMON (n.s.). 



Shell depressed, suborbicular, making about two volutions, with 

 the summit of the spire on the same plane or a little above the 

 plane of the outer volution. Spire small : volutions contiguous 

 throughout their whole extent, very gradually expanding; 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 aggregated in folds or 

 ridges, giving an irregular or undulating surface to the shell. 



This species has the form of Platyostoma ; but the peristome shows 

 no columella, and presents a wide umbilicus. The length of the largest 

 specimen, from the outer margin of the aperture to the opposite side of the 

 volution, is more than three inches : the longitudinal diameter of the aper- 

 ture is nearly two inches ; the width, a little less. 



Geological formation and locality. In the Corniferous limestone : Da- 

 rien, N.Y. I have also seen the same from Port Colborne, Canada West. 



PLATYCERAS DUMOSUM (Conrad). 



P. dumosum : Conrad, Ann. Rep. on the Palaeontology of New-York, 1840, p. 205. 



This shell, in its full size, attains a length of two and a half inches, with 

 the entire surface covered with strong spines sometimes two inches in length. 

 In well-marked specimens these spines are arranged in diagonal rows across 

 the body of the shell, and show a nearly regular quincunx order. One speci- 

 men shows the bases of ninety of these spines, and, from the extent of the 

 shell, the whole number must have been more than one hundred. 



Mr. Conrad's description is as follows : "Shell covered with thick tu- 

 *' bular spines, arranged in longitudinal rows ; margin of aperture waved ; 

 " volutions free." He remarks that " in some varieties the spines are com- 

 paratively few." In some specimens of more than half the full size, there 

 are not more than fifteen or twenty spines ; and in all those with few spines, 

 the expansion of the body-whorl is much less abrupt, while they rarely 

 attain more than half the size of the large tjpical forms. The number of 

 volutions in the smaller forms is fully equal to the larger ones ; being two 

 or more, with the last one quite free. 



Since this form is so distinct and constant, I propose to designate it as a 

 variety. 



1861.] 



