I04 



NORTH AMERICAN INDEX FOSSILS. 



Medium-sized, rapidly expanding, planoconvex; section semi- 

 circular to subtriangular. 



Stones River and Black River beds of Cincinnati region ; Stones 

 River and Trenton of Minnesota. 



130. T. oweni Clarke. (Fig. 1330.) Ordovicic. 

 Gently arcuate, rapidly widening; incurved dorsal side strongly 



and subtriangularly convex. 



Stones River of Minnesota; Black River of Cincinnati region. 



131. T. planidorsatum (Whitfield). (Fig. 1331.) Ordovicic. 



Fig. 1332. Tripteroceras Iambi, with septal view showing siplmncle, X K* (After 

 Clarke, Pal. Minn.) 



Slightly arcuate, gently tapering; incurvation on depressed con- 

 vex dorsal side; venter fiat in middle, curved at side. 

 Trenton of Minnesota and Wisconsin. 



