190 NORTH AMERICAN INDEX FOSSILS. 



prominent. Cardinal areas in both valves. Surface smooth 

 except for concentric lamellae. 



Upper Cambric of Wisconsin, Minnesota, South Dakota, Middle 

 Cambric of Montana, Arizona, Oklahoma. 



Fig. 224. Dicellomus politus exterior natural size, and interior of ventral and dor- 

 sel valves much enlarged. (After Hall.) 



III. Dinobolus Hall. 



Subcircular ; valves convex and thick. Pedicle valve with an 

 acute, prolonged beak; cardinal area triangular, deltidium present. 

 Brachial valve with inconspicuous beak. A V-shaped platform, 

 with the apex of the V anterior, extends in each valve from the 

 beak half way to the front ; the vaults beneath are more conical 

 than in Trimerella. Ordovicic-Siluric. 



6. D. conradi Hall. (Fig. 225, a.) Siluric. 

 Oval, slightly wider than long and very symmetrical in outline. 



About \y 2 inches wide by i^g inches long. 

 Niagaran of Ohio, Iowa, Illinois, Wisconsin. 



IV. Monomerella Billings. 



Gibbous. Umbo of pedicle valve more or less elevated with 

 high cardinal area; umbo of brachial valve inconspicuous. In- 

 ternal platforms of both valves much less prominent than in Trim- 

 erella and Dinobolus and with no empty space beneath ; platform 

 of pedicle valve usually continued forward as a strong septum. 

 Siluric. 



7. M. prisca Billings. (Fig. 225, b-c.) Siluric. 

 Pedicle valve ovate with greatest width at about the anterior 



third of shell ; beak narrowly rounded, front broadly rounded ; 

 septum for about one third the length of shell. Brachial valve 

 about one fourth shorter than the pedicle and more convex, with a 



