24 



concealed l)y <i coating of rock and a crowded mass of Ostrca congesta, and, 



in some specimens, they line the cavity of the shell; the submargin is thicli. 



No nuiscular impression can be traced, nnless the ridged part indicates its 



station. 



" Subgenus Cucullifeka. 



hinge. 



Shell with an upright hood-shaped process on the posterior end of the 

 " H. ECCENTKiCA, Conrad. 



"Ovato-triangular; hinge-line short, very thick; concentric ridges pro- 

 found, six in numlier ; hood strongly and irregularly plicated ; cavity profound. 



" This shell, with the same structure of substance as the preceding, is very 

 unlike it in form, and is represented by one valve only, while a number of tlie 

 preceding species were found. In all specimens of the two forms, the right 

 valve only was obtained. Whether it is allied to the family Bndisk'S of 

 Lamarck is a question I leave for others to decide On the margin of one of 

 tlie valves are attached some small shells, resembling HippurUex, and the fibers 

 of which the shell is composed lie in broken masses on .some valves, and even 

 scattered like piles of pins. 



" The hood of H. exce.ntr'tcus is 2i inches in height, and the height of the 

 valv(! 10 inches ; length, 9 inches. 



"Accompanying these fossils were many specimens of Inoceramus pro- 

 hlematicm, and a fragment of an undetermined species of the same genus." 



IV. PIERRE GROUP. 



In Nebraska and Dakota and Middle Colorado, south of the divide 

 between the waters of tlie Arkansas and Platte Rivers; also, the lower bed 

 of greensand of New Jersey ; Weber River, Wyoming,' below the coal. 

 Besides the numerous remains of reptiles and fishes found in NewJersey, 

 this formation contains saurian (mosasauroid) remains in Colorado. 



V. THE FOX HILLS GROUP. 



Extended i;i Central Dakota ; on the Arkansas and tributaries ; in South- 

 ern Colorado; and as the second greensand bed in New Jersey." 



' Hiiyili'ii's Animal Uepoit, 1S70, ]i. UiT. 



- F(ir ii synopsis of tlie cxtiuct ItcplUia of this ciiocli aud the preceding, see tlie antliois " Extinct 

 liatratliin,"' ic, N. Anieiii^a, I'Inlailclpliiii, lHGO-70. 



