24 



concealed by a coating of rock and a crowded mass of Oslrea congesta, and, 



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



No muscular impression can be traced, unless the ridged part indicates its 



station. 



" Subgenus Cucullifera. 



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



hinge. 



"H. eccentrica, Conrad. 



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

 found, six in number ; 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 the 

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

 valve only was obtained. Whether it is allied to the family Rudistes of 

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

 the valves are attached some small shells, resembling Hippurites, 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. excentricus is 2J inches in height, and the height of the 

 valve 10 inches ; length, 9 inches. 



"Accompanying these fossils were many specimens of Inoceramus pro- 

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



IV. PIERRE GROUP. 



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

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

 of greensand of New Jersey ; Weber River, Wyoming, 1 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 UILLS GROUP. 



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

 ern Colorado; and as the second greensand bed in New Jersey. 2 



'Haydcu's Animal Report, 1870, p. 167. 



i synopsis of tbo extinct Iteptilia of this epoch and tin the author's " Extiuct 



Batrachia," &c, N. America, Philadelphia, 1869-70. 



