TRIASSIC FOSSILS. 23 



nation most strongly. In the largest specimens examined, the 

 lobes occupying the same position are a little the largest, but are 

 very narrow, as are also their corresponding saddles. Their lower 

 extremities are notched, but not dentate. Dorsal lobe unknown. 



Locality : Huinboldt Mining Kegion, Nevada Territory. 



C. WHITNEY!) n. s, 



PI. 4, Figs. 11> 12, 13. 



SHELL of moderate size ; whorls five, enveloping about one-half. 

 Surface of the whorls flattened on the sides for about two-thirds 

 of their width, the dorsal third fceing inclined inwards. Dorsum 

 grooved by a rounded channel, sometimes deep> at others nearly 

 flat. Umbilical margin of the whorls abruptly truncated. Um 

 bilicus broad and shallow; not quite a third of the greatest diame 

 ter of the shell. Sides ornamented by a variable number of 

 arching ribs, which pass from the edge of the umbilical cavity to 

 the margin of the dorsal groove. These ribs are sometimes 

 dichotomous and bear a variable number of nodes. The larsrest 



o 



number is four rows. They are placed, one row at the origin of 

 the ribs, at the edge of the umbilicus ; another row at the dorsal 

 margin ; a third on the middle of the whorl, at the point where 

 the ribs bifurcate ; the fourth row is placed between the last two. 

 The two marginal rows are most constant, although sometimes 

 only the dorsal row remains. The number of ribs is also variable, 

 some specimens bearing twice as many as others of the same size. 

 Occasionally numerous supplementary ribs occur; while in some 

 specimens they are nearly or entirely absent. 



Septum : Dorsal lobe divided at the end into two small, con 

 verging points, by a regularly rounded emargination ; dorsal 

 saddle wider than the lobe and simply curved ; superior lateral 

 lobe terminating in three small tooth-like processes, with an 

 additional one on each side above. Remainder of the septum 

 unknown. The part figured extends about half the width of the 

 whorl. It is taken from a specimen one and a quarter inches in 



