Geol.— Vol. I.] SMITH— COMPARATIVE STRATIGRAPHY. 37 1 



1880. Meekoceras gracilitatis, White, 12th An. Rept. U. S. Geol. Sur. 



Terr. Part I, p. 115, pi. xxxi, figs. 2, a-d. 

 1902. Prionolobiis gracilitatis, F. Freeh, Lethaea Palaeozoica, Bd. II, Lief- 



erung4, p. 631, fig. a. 



Shell compressed, involute, discoidal, deeply embracing, outer whorl con- 

 cealing three-fourths of the inner, and being indented to one-third of the 

 height by the inner whorl. Umbilicus narrow, but open, the width being 

 about one-sixth of the diameter of the shell. The whorl increases rather 

 rapidly in height, the altitude being slightly more than twice the breadth of 

 the whorl, and one-half of the total diameter of the shell. The sides are 

 gently convex from the abruptly rounded umbilical shoulder ; the greatest 

 thickness of the whorl lies at a point even with the top of the inner whorl, 

 thus giving a lenticular appearance to the shell. Venter flattened, biangular, 

 with broad flat space and sharp shoulder angles. 



Surface ornamented with low folds and radial striae of growth, which in 

 age cross the venter in faint corrugations. No true ribs nor spines are ever 

 present. 



Septa ceratitic, saddles all rounded and entire, lobes all serrated, ventral 

 short, divided by a broad shallow siphonal saddle ; first lateral broad and 

 deeper, second lateral narrow as the ventral ; auxiliary series consisting of a 

 short straight row of denticulations following the third lateral saddle, form- 

 ing merely an unindividualized lobe, which, however, is sharply distinguished 

 from the saddle. The inner septa consist of a short divided antisiphonal 

 lobe, with a single lateral. 



The young are much more involute than mature forms, 

 the umbilicus growing wider with age, and the whorls less 

 deeply embracing. The relative width of the umbilicus is 

 variable, also the abruptness of the umbilical shoulder. 



Diener^ thinks that White has confused two species in 

 his figures of Meekoceras gracilitatis, and that plate xxxi, 

 figure lb in White's paper represents a different species 

 from figure 2a. But the difference lies rather in the 

 drawing than in the original specimens. The septa on 

 White's original specimen are as in plate xxxi, figure 2d of 

 White's paper, except that the denticulations are not suffi- 

 ciently marked in the drawing, and the innermost saddle is 

 more sharply defined from the auxiliary denticulations than 

 in the figure. 



Meekoceras gracilitatis White is nearly related to M. 

 boreale Diener, of the Lower Trias of India and Siberia, 

 but differs in not having the auxiliary series individualized, 



1 See Bibliog. 3— p. 132. 



