616 NORTH AMERICAN INDEX FOSSILS. 



Guelph of New York, Canada, 111., Ohio, and Racine beds of 

 Wisconsin. 



44. T. profundus Conrad. Devonic. 

 More strongly embracing, obscurely carinate whorls, with pro- 

 foundly flaring margin, the latter seldom fully preserved. 



Becraft limestone of eastern New York (common). 



XVII. Oxydiscus Koken. 

 Strongly compressed, disciform shells, with mostly slightly 

 embracing, gradually expanding, and sharply-keeled whorls, a 

 lanceolate or subtriangular aperture, without inner callosity, and 

 a deep V-shaped dorsal sinus continued as a long and narrow slit 

 in dorsal keel. Ordovicic-Mississippic. 



45. 0. subacutus Ulrich. (Fig. 826.) Ordovicic. 



Fig. 825. Trematonotus alpheus, FlG. 826. Oxydiscus sub- 



X 2 A* section. ( After Clarke & acutus. ( After Ulrich & Sco- 



Ruedem.) field, Pal. Minn.) 



Volutions embracing one third to one half of the preceding, 

 acutely carinated ; section of whorl subtriangular, abruptly in- 

 flected at umbilicus. 



Trenton of Kentucky, Tennessee (?), and Minnesota (?). 



46. 0. curvilineata (Conrad). (Fig. 827.) Devonic. 

 Whorls embracing about half the preceding one ; sides rounded, 



rather abruptly inflected at the umbilicus, sinus about one fourth 

 of a volution ; striae of growth often in fascicles ; keel sharp. 

 Oriskany, Schoharie, and Onondaga of New York. 



47. 0. cryptolites (Hall). Mississippi. 

 Whorls more embracing than in preceding species, so as to leave 



only a small umbilicus ; less compressed ; carina less sharply 

 marked ; aperture subtriangular. 



