"90 PROCEEDINGS OJ THE NATIONAL MUSEUM vol. 78 



Description. — Carapace large, relatively short, compressed convex; 

 outline, excepting the antero-dorsal angle and the straight hinge, 

 nearly circular ; height a little less than four-fifths the greatest length , 

 postdorsal angle very obtuse, anterior angle sharply defined, 85° to 

 90° ; hinge line straight, its length nearly equaling the greatest height 

 of carapace; anterior margin approximately straight and nearly 

 vertical in the upper two thirds. Valves moderately convex, thickest 

 in the antero-dorsal half in which part the slope to the outer edges is 

 less gentle than toward the ventral and posterior sides. Test un- 

 known, replaced in the specimens by a ferruginous coating of the 

 interior cast. 



Dimensions: Length 17.1 mm.; height, 14.2 mm.; length of hinge, 

 13.5 mm. Other specimens referred to this species range in length 

 between 8 and 12 mm. 



Remarks.— This species is founded chiefly on two opposite valves, 

 both of which were somewhat poorly illustrated by Walcott and 

 referred by him ^° to Nothozoe vermontana Whitfield. New figures, 

 based on photographic prints of the originals, have been prepared for 

 this work. Comparing these with Whitfield's figures of A^^. vermon- 

 tana, it will be observed that the former are quite different from the 

 latter, especially in the outline of the dorsal half. 



While there is considerable reason for the retention of Whitfield's 

 original specimens under Nothozoe, a similar course in the case of the 

 specimens above described is manifestly unjustifiable. Their refer- 

 ence to Dielymella, though not entirely satisfactory, is yet within the 

 range of a reasonable extension of the boundaries of this genus. The 

 dominant external characteristics of the carapace of Dielymella such 

 as the rectangular form of the antero-dorsal part of the outline, the 

 straight hinge, the general curvature of the posterior margin, and 

 the fullness of the anterior part of the surface, are all present in D. 

 brevis. The habitat of D. brevis also seems to have been the same as 

 that of all other species of the genus. It occurs in a coarse, quartzitic 

 sandstone lithologically very similar to the bed of sandstone from 

 which the preceding Arizona species were procured. 



Compared with other species of Dielymella, apparently the principal 

 and only strildng difference is in a feature that, under ordinary cir- 

 cumstances, can not be given a greater significance than specific, 

 namely, in the matter of relative length, this being greatly less in 

 D. brevis than in the other forms referred to the genus. 



Occurrence. — From a bowlder of quartzite found on Sunset Hill, 

 near Lake Dunmore, Salisbury, Vt., and presumably derived from 

 nearby Lower Cambrian ledges. 



Coty pes. —Csit. No. 56504, U.S.N.M. 



" loth Ann. Kept. U. 8. Qeol. Surv., 1890, pi. 80, figs, ia, 46. 



