582 Professor Emerson on Hall's Geological Collections. 



rior jirominent, projecting slightly over the hinge-line and running downwards and 

 backwards ; the posterior rising abruptly from the broad sulcus and sloping, with 

 flat surface to the point of junction of the dorsal and posterior border. The 

 broad sulcus is hollowed between these two tubercules, j»asses around them 

 anteriorly and posteriorly, and is bounded on the outside by 

 a ridge which starts at the front side of the anterior tubercle 

 ^'^ and arches round till it coincides in direction with the ventral 



9 1 margin, which it then follows to the posterior portion of the 



Fig. 9 a, right valve, valve, where it curves round sharply to meet the posterior side 



cas o m enoi, ,>, ^^ ^^^^ posterior tuborcle. This ridge is highest in the middle, 



fig. 9 0, end view. ^ & & ? 



Length, 2fm m. ; and there sharply elevated and bent slightly toward the dorsal 



breadth, u mm. margin, while at both ends it is flatter and less distinctly 



marked off from the rest of the valve. It is separated from the ventral rim by a 



deep, regularly concave groove, which becomes broader and ill-defined towards 



the ends of the valve. The whole valve remotely resembles the cast of a bivalve 



shell with abnormally deep and large pallial and muscular impressions. 



The description is made from two sharply marked casts, the shell being 

 present only in the ventral furrow, where it is thick, smooth, and light brown. 



Found in the gray limestone with other Entomostraca. 

 Triaethrus Beckii, Green. 



The separated heads of this species are very abundant in the black lime- 

 stones, and the separated cheeks and tails are also i^resent in great numbers. 

 They are all small, the heads being 4-7 mm. in length. The occurrence of this 

 common Utica slate species so far north is interesting, and, taken with its appear- 

 ance in the Trinuclid Skiffer of Sweden (Liunarsson, loc. cit., p. 70, fig. 27), where 

 it is also of the same small size as here, gives it a wide distribution. 



CALY3IENE SENARIA, Courad. 



Several small, flattened, well-preserved tails, and one cheek, which belonged 

 to individuals not more than 12-15 mm. long, occur in the black limestone with 

 the other Utica slate fossils enumerated. 

 Phacops, sp. 



The pustulose elevated glabella of a small individual has the shape common 

 to species of this genus. 



In black limestone. 

 ASAPHUS, sp. 



A stout spine, triangular in cross section, and niiukcd on tlie under side 

 exactly as in Asaidms gigas. 



Ill ;;iav liiiit'stoiM' witli iMitoinostraca. 



