74 



NORTH AMERICAN INDEX FOSSILS. 



XXV. Sphyradoceras Hyatt. 



Costated trochoceracones, the last whorl often free ; costae as in 

 the preceding, often with faint longitudinal ridges. Siluric- 

 Devonic. 



yy. S. desplainense (McChesney). (Fig. 1286.) Siluric. 



Often large, dextrally coiled; costae curving gently backwards, 

 rounded. 



Niagaran and Guelph of Wisconsin; Guelph of New York, 

 Ontario, Illinois, Indiana, etc. 



78. S. costatum (Hall). (Fig. 1287.) 



Siluric. 



Fig. 1286. Sphyradoceras desplai- 

 nense, X Vi' ( After Hall, 20th Mus. 

 Rep.) 



Fig. 1287. Sphyradoceras costatum, 

 from below, >( Vz- (After Hall, 20th 

 Mus. Rep. ) 



Sinistrally coiled ; whorls less rapidly enlarging than preceding ; 

 costae somewhat sigmoid, sharper and narrower. 



Racine and Guelph of Wisconsin ; Guelph of New York, Ohio, etc. 



79. S. Clio (Hall). (Fig. 1288.) Devonic. 



Sinistrally and more closely coiled than preceding, forming 

 higher spire ; costae closer and rounded, slightly sigmoidal, rather 

 faint; longitudinal striae in well preserved specimens. 



Schoharie of New York, Michigan, etc. 



XXVI. MiTROCERAS Hyatt. 



Trochoceracones coiling in a high, turbinate spire of gradually 

 enlarging whorls, with deep umbilication. Non-costate. Siluric. 



