ECHINOCONCHUS 139 



the greatest width, the cardinal extremities obtusely angular or a little 

 rounded. The dimensions of a nearly complete example are: length 

 from hinge-line to front margin 42 mm., length from umbonal region to 

 front margin 50 mm., greatest width 49 mm., length of hinge-line 40 

 mm., convexity of pedicle valve 25 mm., depth of visceral cavity between 

 the two valves 18 mm. 



Pedicle valve strongly convex with the point of greatest convexity 

 posterior to the middle, the surface curving abruptly to the cardinal 

 margin, less abruptly to the lateral margins and more gently to the 

 anterior margin, compressed towards the cardinal extremities in sub- 

 auriculate expansions which are not sharply differentiated from the 

 general surface, the umbonal region prominent and strongly protuberant 

 beyond the cardinal margin ; mesial sinus originating in the umbonal 

 region, usually of moderate depth, rather broad, rounded in the bottom 

 and ill defined laterally; the beak strongly incurved. Surface of the 

 valve, in the exfoliated condition, marked by strong, regular, concentric 

 bands which are rather abruptly elevated at their anterior margin with 

 the surface sloping regularly to the line of elevation of the next 

 posterior band, these bands vary in width from 1 to 5 mm., usually being 

 narrower towards the beak and again towards the front margin in 

 mature shells, with the broadest bands in the middle portion of the valve ; 

 when the surface is not exfoliated these concentric bands bear closely 

 crowded, fine, appressed spines directed anteriorly, the more posterior 

 rows overlapping those in front, upon each concentric band the coarser 

 spines are along its anterior margin, the more posterior ones being finer, 

 with a narrow, naked area along the posterior border of each band nearly 

 or wholly free from spines. 



Brachial valve concave at the beak, the concavity extending anteriorly 

 with broadly diverging, but indefinite lateral borders which intersect the 

 lateral borders of the valve at about mid-length, towards the 'cardinal 

 extremities the surface is somewhat flattened, the visceral portion not 

 sharply differentiated from the produced part ; near or posterior to the 

 middle of the valve, a low, but rather broad, rounded, ill-defined mesial 

 fold originates which grows more prominent anteriorly. Surface marked 

 by concentric bands and fine, appressed spines similar to those of the 

 opposite valve. 



Bemarks.It is not clear that this species should be differentiated from 

 E. punctatus Martin, but this form having its most typical development in 

 the Keokuk limestone commonly has been considered as distinct from 

 the Pennsylvanian shell usually designated by Martin's name. The 

 Keokuk shell has a proportionally longer hinge-line, a broader umbonal 

 region and more obtusely pointed beak than the Pennsylvanian shell, 

 and the concentric bands of the surface are usually broader and stronger, 



