604 NORTH AMERICAN INDEX FOSSILS. 



2. T. convexum Berkey. (Fig. 802, c, d.) Cambric. 

 Larger than preceding, lower, and with more broadly ovate- 

 acuminate aperture, and anterior beak. 



Associated with the preceding. 



3. T. barabuensis Whitfield. (Fig. 802, e t f.) Ordovicic. 

 More narrowly ovate than preceding ; beak slightly overhang- 

 ing ; anterior slope concave. 



In the Lower Magnesian series (Jordan sandstone) of Wis- 

 consin. 



4. T. nycteis (Billings). (Fig. 803.) Ordovicic. 



Fig. 803. Triblidium nycteis. (After Billings. ) 



Larger than the preceding, and with the beak curved down- 

 wards. 



Beekmantown of Mingen Islands. 



II. Helcionopsis Ulrich and Scofield. 

 Like Triblidium, but surface marked by fine radiating striae. 

 Ordovicic-Siluric. 



5. H. striata Ulrich. (Fig. 805, a, b.) Ordovicic. 

 Ovate, rather narrow in front, apex incurved and slightly pro- 

 jecting ; radiating lines round, equal. 



Upper Cincinnatian : Kentucky and Ohio. 



III. Hypseloconus Berkey. 

 High conical shells, differing from Triblidium in being curved 

 towards the broader side of the shell. Cambric-Ordovicic. 



6. H. recurvus (Whitfield). (Fig. 804, a, b.) 



Cambric-Ordovicic. 



Curvature very slight, not beyond posterior margin ; shell high ; 

 angle somewhat variable. 



St. Croix (Upper Dresbach) and Lower Magnesian of Minnesota 

 and Wisconsin. 



