114 BULLETIN UNITED STATES GEOLOGICAL SURVEY. [YolY. 



each, irregular in size and sliape, but all much smaller than the lateral 

 lobes and cells; the auxiliary lobes much resembling the digitations of 

 the lateral lobes. Surface in adult shells shoAAing the usual lines of 

 groAvth and strong transverse Arrinkles, some of A^^hich assume a slight 

 degree of regularity as transverse ridges ; but in the case of the young 

 these ridges, although not prominent, are distinct, and almost the same 

 as in the young of 31. aplanatum. ^ 



Diameter of the coil of the largest example in the collection, 118 mil- 

 limeters ; that of a smaller, but more perfect one, 95 millimeters ; 

 breadth or vertical diameter of the outer volution of the same, 43 milli- 

 meters ; transA^erse diameter of the same, 22 millimeters. 



Position and locality. — Jura-Trias strata ; member D of the foregoing 

 section at locality No. 1, Southeastern Idaho. Collected by Dr. A. C. 

 . Peale and J. E. Mushbach. 



The folloAving are Professor Hyatt's remarks upon this species : 

 " This species differs from "A" [3£ee1coceras a/planatimi] in never having 

 a flattened abdomen. This is rounded. The sides in the young and the 

 aspect of the shell are precisely the same as in ''A" [ill. a])lana.tum], but 

 the increase of the Avhorls by groAvth is much more rapid, so that at the 

 diameter of an inch, or even someAvhat less, they begin to flatten on 

 the sides, shoA7 an abrupt umbilical shoulder, and cover up one-third 

 of the A7horl by involution. In consequence of the rounding of the abdo- 

 men, hoAvever, the sides actually appear in the full-groA\^n to be more 

 gibbous than in "A" [ill. aplanatum]. One specimen at the total diameter 

 of two and a half inches has one-half of the adjacent internal A^'horl cov- 

 . ered by the outer one, an amount of iuA^olution Avhich occurred much 

 , earlier in the history of the shell than at the diameter measured. Large, 

 . coarse folds are present in some of the specimens. i 



"This species in some cases has a strong resemblance to Arcesfesf 

 .perplanus Meek, but is less iuA^olute." (A. H.) 



Meekocekas gracilitatis White.— Shell depressed-discoidal or 

 sublenticular 5 siphonal side of the A^olutions flattened, the flattening 

 being \^ery distinct upon the inner A'olutions, even upon the smaller 

 ones, but sometimes less so upon the living-chamber of fuU-groAYU 

 specimens ; umbilicus shalloA\^ in consequence of the slight transA^erse 

 diameter of the shell, but it is somcAvhat open, although narroAver than 

 that of either of the preceding species, its Avidth in the adult being not 

 more than one-half the greatest vertical diameter of the outer A'olution, 

 and in the young it is proportionally much less ; A^olutions flattened- 

 convex on their sides, rounded someA^•hat abruptly inA^^ard to meet the 

 next volution Avithin, but they are not so distinctly shouldered there 

 . as in the tAvo preceding species, except perhaps in the younger of the 

 A'olutions ; the amount of iuA^olution being so great in the young sliell 

 that the earliest A'olutions are nearlj^ or quite coA^ered, but it so dimin- 

 ishes Avith the groAvth of tlie shell that in fully adult specimens the outer 

 A'olution does not embrace more than one-half the width of the next 



