66 



NORTH AMERICAN INDEX FOSSILS. 



center. Septa deeply concave, with usually ventral and dorsal 

 saddles and lateral lobes. Surface smooth or slightly costated. 

 Ordovicic-Siluric. 



60. B. natator (Billings). (Fig. 1274.) Ordovicic. 



Whorls slender, compressed, oval in sections; siphuncle extra- 



FiG. 1274. Barrandeoceras natator, section showing septa partially preserved, X K* 

 (After Ruederaann, Bull. 90, N. Y. State Mus.) 



centroventran ; no contact furrow; surface costated except in 

 adult. 



Chazy limestone of Lake Champlain region. 



61. B. convolvans (Hall). Ordovicic. 



Section of whorls much compressed, oval; siphuncle centroven- 

 tran to intracentroventran. Surface smooth. 



Black River of New York. 



XIX. Tarphyceras Hyatt. 



Discoidal, with gradually enlarging whorls in contact and slightly 

 impressed. Last whorl often diverging. Siphuncle ventran ii) 



