114 GEOLOGICAL SURVEY OF THE TERRITORIES. 
MEEKOCERAS MUSHBACHIANUM White. 
Plate 32, figs. 1 a, }, c, and d. 
Meekaceras mushbachanus White, 1879, Bull. U. S. Geol. Sur. Terr., vol. v, p. 113. 
Shell compressed-discoidal, having the peripheral side abruptly 
rounded; umbilicus open, shallow, about two-thirds as wide as the full 
breadth of the outer volution in the adult and proportionally narrower 
in the young; volutions flattened-convex on their sides, the convexity 
merging gradually into that of the peripheral side, but upon the umbili- 
cal side the volutions, especially the outer one, are abruptly shouldered ; 
volutions in the young shell distinctly embracing, but the amount of 
involution increasing with its growth, so that the outer one embraces 
nearly one-half of the next adjacent one. Siphonal cell of the septa, at 
or near adult size of the shell, small, slightly notched atthe end; middle 
and outer lateral cells of nearly equal size, a little larger than either uf 
the lobes, except in some cases the middle one, somewhat regularly 
rounded in front; inner lateral cell smaller than either of the others, its 
inner anterior border a little more abruptly rounded than elsewhere, but 
the inner side not defined for want of another lobe; middle and outer 
lateral lobes nearly. equal in size, but the former a little the longer; each 
bearing from six to eight prominent narrow digitations; the inner lateral 
lobe about half as wide as the outer lateral one, but of similar length, 
and bearing three or four digitations; auxiliary lobes and cells between 
theinner lateral cell and the inner edge of the volution indistinctly defined, 
but adjacent to the inner lateral cell there are three or four digitations 
such as mark the ends of the lobes, between which and the inner mar- 
gin of the volution the edge of the septum merely deseribes a flexed or 
sigmoid line. This line and the adjacent serrations seem to represent 
respectively an incipient lobe and cell which, if developed, would be 
co-ordinate with the principal lobes and cells. Surface in adult shells 
showing the usual lines of growth and strong transverse wrinkles, 
some of which assume a slight degree of regularity as transverse ridges; 
put in the case of the young shell these ridges, although not prominent, 
are distinet, and quite similar to those in the young of J. apalantum. 
Diameter of the coil of the largest example in the collection, 118 
millimeters; that of a smaller but more perfect one, 95 millimeters; 
breadth, or vertical diameter, of the outer volution of the same, 43 
mnillimeters; transverse diameter of the same, 22 millimeters. 
Position and locality—Triassie strata; member D of the section 
shown on a previous page of locality No 1, Southeastern Idaho, where 
it was collected by Dr. A. C. Peale and Mr. J. E. Mushbach. 
The following are Professor Hyatt’s remarks upon M. mushbachanum: 
“This species differs from ‘A? [J/. aplanatwm| in never having a flat- 
tened abdomen. This is rounded. The sides in the young and the 
aspect of the shell are precisely the same as in ‘A’ [J aplanatum|], but 
the increase of the whorls by growth is much more rapid, so that at the 
diameter of an inch, or even somewhat less, they begin to flatten on the 
sides, show an abrupt umbilical shoulder, and cover up one-third of 
the whorl by involution. In consequence of the rounding of the abdo- 
men, however, the sides actually appear, in the full-grown, to be more 
gibbous than in ‘A’ [M. aplanatwm]|. One specimen at the total diam- 
eter of two and a half inches has one-half of the adjacent internal whorl 
covered by the outer one, an amount of involution which occurred much 
earlier in the history of the shell than at the diameter measured. Large, 
coarse folds are present in some of the specimens. 
“This species in some cases has a strong resemblance to Arcestes? 
perplanus Meek, but is less involute.”—(A. H.) 
