192 



NORTH AMERICAN INDEX FOSSILS. 



10. T. grandis Billings. (Fig. 226, c d.) Siluric. 



Ovate, with the greatest width a little in advance of the middle. 

 Both valves moderately and uniformly convex. Surface marked 

 with obscure concentric growth lines. Length of a large shell is 

 3 inches; width, 2% inches. This species differs from T. acumi- 





brachial valve, X % 5 c > Trimerella grandis, internal mold viewed from brachial 

 side, y^%; d, the reverse of the same specimen, X %• («> deltidium ; c, deltidial 

 ridges; d, areal borders ; i, umbonal chambers ;J, platform ; k, platform vaults ; m, me- 

 dian scars ; q, crown of crescent ; r, side of crescent.) (After Hall and Clarke.) 



nata in that the tubular cavities of the interior do not extend to the 

 beak, the pedicle valve is broader and the concentric lines are 

 obscure. 



Guelph of Ohio, Wisconsin, Ontario. 



VI. Lingulella Salter. 

 Valves Lingula-Y(kz but pedicle valve pointed, with a cardinal 

 area, and elevated beyond the brachial valve ; a distinct pedicle 

 groove present. Cambric-Ordovicic. 



