68o 



NORTH AMERICAN INDEX FOSSILS. 



Fig. 952. Diaphorostoma ventr 



(After Hall.) 



233. D. lineatum (Conrad). (Fig. 953.) Devonic. 



Regularly enlarging whorls, forming moderate spire ; scarcely 

 depressed sutures ; fine regular spirals and transverse striae. 



Onondaga and Hamilton of New York, Indiana, and elsewhere 

 in eastern North America. 



Fig. 953. Diaphorostoma lineatum. (After Hall.) 



LXXIV. Platyceras Conrad. 

 Young shell coiled as in Diaplwrostoma, late stages non-coiling, 

 often spinous. (In this polyphyletic genus are 

 commonly included non-coiling shells of di- 

 verse origin ; the typical species are derived 

 from Diaphorostoma.) Sil uric-Carbonic. 



234. P. niagarense (Hall). (Fig. 954.) 



Siluric. 



Apex only enrolled in one volution ; re- 

 mainder rather rapidly expanding, last portion 

 longitudinally undulated. 



Niagara of New York and elsewhere. 



235. P. gebhardi Hall. (Fig. 955, 956.) 



Devonic. 



Like Diaphorostoma ventricosa, but loose- 

 coiling, last whorl partly free. 



Fig. 954. Platyceras 

 niagarense. ( After 

 Hall.) 



