WHITE. CRETACEOUS FOSSILS. 35 
moderately convex, increasing uniformly in size; last volution not ex- 
panded disproportionately with the other volutions, tapering gradually 
forward into a moderately long and somewhat stout beak, which is a 
little deflected to the left; aperture narrow, lance-ovate in outline, 
acutely angular at its distal end, and ending anteriorly in the canal 
formed by the beak before mentioned; columella nearly straight, its 
folds not seen; outer lip gently sinuous, its general direction approxi- 
mately parallel with the axis of the shell. Surface, especially that of 
the last volution, marked by strong lines of growth and also by faint 
revolving lines. The distal border of the volutions is also marked by a 
ridge or narrow shoulder adjacent to the suture, which has the appear- 
ance of consisting of one or more strongly raised revolving lines. 
Length, 80 millimeters; breadth of last volution, 20 millimeters; 
length of aperture, including canal, 33 millimeters. 
This species is closely related to F. (P.) culbertsoni Meek & Hayden, 
but it differs in the much greater proportionate length of the spire, and 
in the elevation or shouldering of the distal border of the volutions, 
upon the proximal side of the suture. 
Position and locality—Cretaceous strata, valley of Yellowstone River, 
Montana, where it was collected by Mr. J. A. Allen, and in whose honor 
the specilic name is given. 
CEPHALOPODA. 
Genus PRIONOCYCLUS* Meek. 
PRIONOCYCLUS WYOMINGENSIS Meek. 
Plate 15, figs. 1 a, 6, c, d, and e. 
Ammonites ( Pleuroceras) serrato-carinatus Meek, 1871, An. Rep. U.S. Geol. Sur. Terr. for 
1870, p. 298 
Prionocyclus w yomingensis Meek, 1876, U. 8. Geol. Sur. Terr., vol. ix, p. 452. 
Shell discoidal; umbilicus broad; volutions very slightly embracing 
or sometimes merely in contact, comparatively slender, the vertical di- 
ameter of the outer volutions about one-third greater than the trans- 
verse diameter, but this difference between these ‘dimensions of the inner 
volutions is greater than that of the outer; outer sides of the volutions 
flattened, convex, and abruptly rounded to both the peripheral and um- 
bilical sides, the former side bearing along the median line a sharply- 
raised, strong carina, the edge of which is studded with more or less 
distinct small tubercles or serrations; sides of the volutions marked with 
numerous transverse sharply-raised cost, which become obsolete upon 
the umbilical side, and upon reaching the peripheral side they all bend 
abruptly forward, and become obsolete before quite reaching the carina. 
These costz are small and of nearly uniform size in very young exam- 
ples, but they gradually increase in size with the growth of the shell, 
and a part of them become stronger than the others, and usually every 
third or fourth costa is strengthened towards its inner end, and occa- 
sionally towards its outer end also, by the coalescence there with the 
adjacent costa in front of it. 
At the points of the coalescence and also upon some others of the 
larger cost, more or less distinct but somewhat irregular tubercles are 
*For generic diagnosis. of this genus and a subgenus Prionotropis, see U. S. Geol. Sur. 
Terr., vol. ix, pp. 452-455 
