BRACHIOPODA—ATREMATA. 



191 



more broadly rounded anterior portion. Pedicle valve one and a 

 half inches long by a little over an inch wide. 

 Guelph of Ohio, Illinois, Ontario. 



Fig. 225. a, Dinobolns conradi, interior of pedicle valve, X 2 A { a > deltidium ; 6, 

 deltidial slope ; c, deltidial ridges ; d, areal borders ; r, side of crescent muscular area) 6, 

 Monomerella prisca, internal mold of brachial valve ; c, internal mold of pedicle valve, 

 X %■ {h umbonal chambers ; k, platform-vaults ; m, median scars ; n, anterior scars; 

 0, lateral scars ; r, side of crescent. ) (After Hall and Clarke.) 



V. Trimerella Billings. 



Thick, elongate-ovate. Cardinal area of pedicle valve very 

 prominent ; pedicle opening closed by a broad deltidium which is 

 concave owing to the bounding ridges. Brachial valve with no 

 cardinal area but incurved against the deltidium. Long and 

 narrow platforms are present in both valves extending from the 

 apex over half way to the anterior margin ; that of the brachial 

 valve is the higher. The area beneath the platforms is occupied 

 by two long, tubular cavities. Most American specimens are 

 internal molds in dolomitic limestone. Siluric. 



8. T. acuminata Billings. Siluric. 

 Pedicle valve ovate, widest a little in advance of the middle, 



tapering from there with nearly straight sides to the beak which 

 is almost acute ; the platform bears a deep median furrow. 

 Brachial valve much shorter than the pedicle, more convex and 

 with beak more strongly incurved. Surface marked by coarse 

 concentric growth lines. A very large specimen measures 3*^ 

 inches in length by 3 inches in width. 

 Guelph of Ohio, Illinois, Ontario. 



9. T. ohioensis Meek. (Fig. 226, a, b.) Siluric. 

 Differs from T. acuminata in its broader form, and less produced 



and blunter beak, with thicker septa as indicated by the mold. 

 In the Niagaran beds of Ohio, Illinois and Ontario. 



