CEPHALOPODA— ORTHOCHOANITES. 73 



costae curving backwards on the side ; siphuncle ventral of center. 

 Ordovicic-Siluric. 



74. P. jason (Billings). (Fig. 1285.) Ordovicic. 

 Nautilicones with faintly impressed whorls gradually enlarging; 



costae separated by broader concave spaces, which are finely striated 

 parallel to the costae; siphuncle large, propioventran. 



Chazy of Mingen Island, Newfoundland and the Lake Cham- 

 plain region. 



75. P. bondi (Safford). Ordovicic. 



Fig. 1285. Flectoceras jason, X %> shelf with inner whorls broken away. (After 

 Ruedemann, Bull. 90, N. Y. State Mus. ) 



Loosely coiled gyroceracones, with rapidly enlarging whorls of 

 oval section. Costae sharp, strong, separated by broad concave 

 interspaces. 



Stones River of Minnesota (?) and Tennessee; Stones River 

 and Black River of Cincinnati region. 



y6. P. bickmoreanum (Whitfield). Siluric. 



Whorls forming an open gyroceracone, the last whorl in gerontic 

 individuals becoming free and sometimes completely straightened 

 out. 



Niagaran of Indiana. 



