CRETACEOUS FOSSILS. 63 



Septum : Dorsal lobe small, not more than a third as long as 

 the superior lateral; divided for half its length into two bifur 

 cated branches with a small spur above on each side. Dorsal 

 saddle broad, oblique, divided into two unequal branches, each 

 of which is again subdivided. Superior lateral lobe broader 

 than the dorsal saddle, one large branch at the end with five 

 alternating spurs; on the dorsal side of this there is a very large 

 oblique branch with several smaller branchlets ; the corresponding 

 branch on the ventral side is much smaller, and has but three 

 terminal spurs. Lateral saddle very oblique, smaller than the 

 dorsal, and divided into three bifurcate branches. Inferior 

 lateral lobe narrower than the superior lateral, with a terminal 

 branch, and two large and two small alternating spurs on each 

 side. The first supplementary lobe is not more than half the 

 size of the preceding one. The rest, two more, decrease regu 

 larly in size. 



Figures 8 and 8 , are two-thirds of the natural size. 



Locality: North fork of Cottonwood Creek, Shasta County. (Div. A.; 



Named after Dr. F. V. Hayden, to whom we are indebted for so large a propor 

 tion of our knowledge of the geology of Nebraska and the neighboring Terri 

 tories. 



A. PERUVIANUS, De Buch? 



PI. 10, Fig. 9. 



The fragment figured is referred with doubt to this species. It is the only speci 

 men yet found, and was collected at Tuscan Springs, Tehama County, by Dr. J. 

 A. Veatch. The ribs have almost exactly the character of those of the specimen 

 iigured by Marcou, Geol. of N. America, PI. 5, Fig. 1 a. 



A. TRASKII, n. s. 



PI. 11, Fig. 10; PI. 12, Fig. 11. 



SHELL robust. Whorls six or more, embracing about one- 

 half; rounded on the sides and back. Umbilicus broad and 

 deep, about a third of the diameter of the shell, sides abrupt, 

 but uniting with the surface by a regular curve. Surface marked 



