WHITE.] TRIASSIC FOSSILS, 115 
MEEKOCERAS GRACILITATIS White. 
Plate 31, figs. 2, a, b, ¢, and d. 
Meekoceras gracilitatis White, 1879, Bull. U. S. Geol. Sur. Terr., vol. v, p. 114. 
Shell compressed-discoidal or sublenticular; siphonal side of the 
yolutions flattened, the flattening being very distinct upon the inner 
volutions, even upon the very small ones, but sometimes less so upon the 
living-chamber of full-grown specimens; umbilicus shallow, in conse- 
quence of the small transverse diameter of the shell, but it is somewhat 
open, although narrower than that of either of the two species just 
described, its width in the adult being not more than one-half the 
ereatest vertical diameter of the outer volution, and in the young it is 
proportionally much less; volutions flattened-convex on their sides, 
rounded somewhat abruptly inward to meet the next volution within, 
but they are not so distinctly shouldered there as in the two preceding 
species, except perhaps in the young volutions; the amount of involu- 
tion being so great in the young shell that the earliest volutions are 
nearly or quite covered, but itso diminishes with the growth of the shell 
that in fully adult specimens the outer volution does not embrace more 
than one-half the width of the next one within. . 
Siphonal cell of the septa, near the outer portion of an adult example, | 
small; the outer, middle, and inner lateral cells larger, and both of 
nearly equal width; the outer cell regularly rounded at the end; the 
inner one more abruptly rounded at the inner side of the end than 
elsewhere; the inner cell not defined. upon its inner side for want 
of another full lobe there, but its width between the auxiliary series 
and the inner full lobe. is nearly as great as that of the other cells; 
ventral or outer lobe small, bearing two or three small digitations at its 
end; middle lobe largest, its width being nearly equal to that of the 
lateral cells, bearing at its rounded extremity five or six small digita- 
tions of nearly uniform size; inner lateral lobe small and narrow, bear- 
ing two or three, rarely four, small digitations at its end; auxiliary 
lobes and cells occupying a space adjacent to the next inner volution 
about as wide as that of the outer lateral lobe, and constituting a finely- 
undulated or serrated suture. In the more distinct examples the 
lobes appear minute and pointed, and the cells a little larger, and 
rounded at their ends, the one adjacent to the inner border being a 
little larger than the others. Surface of young examples nearly or quite 
plain, but in fully adult shells there is a tendency to form nodes or ribs, 
the latter sometimes crossing the periphery; but they do not appear to 
assume that regularity which we find in typical Ceratites. 
Diameter of the coil of the largest example in the collection, 100 milli- 
meters; vertical diameter of the outer portion of the living-chamber, 45 
millimeters; transverse diameter of the same, 20 millimeters. 
Position and locality —Triassic strata; member D of the section at lo- 
cality No. 1, shown on a previous page, Southeastern Idaho, where it 
was collected by Dr. A. C. Peale. 
The following are Professor Hyatt’s remarks upon M. gracilitatis: 
“This species differs from ‘B’ [AL mushbachianum] in about the same 
way that ‘B’ differs from ‘A’ [M. aplanatum], except in so far as it ap- 
proximates more closely to ‘A’ in having a similar flattened abdomen, 
This flattened abdomen appears at a much earlier age than in the less 
involute form ‘A.’ In fact, before the shell reaches the diameter of 
three-sixteenths of an inch, not only is the abdomen flattened, but the 
sides also; and the increase by growth is so rapid that the sides of the 
