Geol.— Vol. I.] SMITH— COMPARATIVE STRATIGRAPHY. 38 1 



of that genus. The foregoing description is based partly 

 on Meek's type specimen, which was imperfect, but chiefly 

 on perfect specimens of this and one other species, col- 

 lected by the writer in the West Humboldt Range, Nevada. 



This genus has been extended to take in the group of 

 Ammonites sandlingensis Hauer of the Mediterranean 

 region, but all those species have ammonitic septa, usually 

 a hollow keel, and long body-chamber, and so fall under 

 the Tropitidee, but they are left for the present with Eido- 

 moceras. 



Eutomoceras s. str. is confined to the Middle Trias, in 

 which horizon it is rather common in the West Humboldt 

 Range of Nevada, in the Daonella beds, associated with 

 numerous nodose ceratites. 



Eutomoceras dunni Smith, sp. nov. 



Plate XLIII, Fig. 11 ; Plate XLIV, Fig. 4- 



Shell involute, discoidal, laterally compressed. Whorls high, deeply em- 

 bracing, and rather deeply indented by the inner volution. Sides slightly 

 convex, sloping from the greatest breadth at the umbilical shoulder to the 

 narrow acute venter, with abruptly rounded abdominal shoulders. Venter 

 surmounted by a high keel, without marginal furrows. Umbilical shoulders 

 abruptly rounded, umbilicus narrow and deep. The height of the whorl is one- 

 half the total diameter of the shell, and the width is two-thirds of the height; 

 it is indented to one-fifth of the height by the inner whorl. The width of 

 the umbilicus is one-fifth of the diameter of the shell. The surface is orna- 

 mented with coarse ribs that bifurcate from coarse knots on the umbilicus, 

 and branch again about one-third of the way up the flanks, then curve 

 sharply forward on the abdominal shoulders to the base of the keel. There 

 are frequent knots on the lateral ribs, but these are not arranged in regular 

 spiral rows, and the intercalary ribs do not fork on the flanks. 



The septa are ceratitic, with rounded entire saddles and serrated lobes, 

 like those of Eutomoceras laubei Meek. 



Eutomoceras duntii is nearly related to Eutomoceras 

 laubei with which it is associated, but differs in its more 

 robust whorl, stronger sculpture, higher keel, and slightly 

 narrower umbilicus. 



The dimensions of the type specimen, figured on plate 

 xliv, figure 4, and plate xliii, figure 11, are as follows: 



