5 
III conclusion Meek says: 
“Should complete undistorted specimens show as I think very probably will be the 
case, that it differs generically or subgenerically from Phylloceras proper, I would propose 
for the group into which it would in that case naturally fall, the name Rhaeboceras.” 
Rhaeboceras halli Meek 
Ammonites halli 
F. B. Meek and F. V. Hayden: Acad. Nat. Sci., Phil. Proc., vol. VIII 
(1856), p. 7; vol. XII (1860), p. 420. 
F. B. Meek: Smith. Check-List of North American Cret. Fossils 
(1864), p. 24, Fig. 64. 
Phylloceras f halli 
F. B. Meek: U.S. Geol, Surv., Terr., vol. IX (1876), pp. 458-462, 
PI. 24, figs. 3a, b, c. 
Description from Meek: 
“Shell attaining a large size, moderately compressed discoidal; volutions with their 
convexity about equalling two-thirds their diameter from the ventral side to the rather 
narrowly rounded periphery, in young and medium sized examples, each embracing nearly 
the entire breadth of the next within, but the last one in the adult becoming proportionally 
less deeply embracing; umbilicus very narrow, and rather deep in the young but propor- 
tionally wider in the adult; surface ornamented with numerous small bifurcating, sligMly 
flexuous costae, that are larger near the umbiUcal side, and on the la.st turn of medium 
and large sized specimens become proportionally somewhat more prominent, more curved, 
and suddenly bifurcate near the umbilicus, and again divide and subdivide into num- 
erous smaller ones, so that their number, including others intercalated betweenj amounts 
to from five to seven times as many where they pass straightly over the periphery, as 
near the umbilical side; body-chamber forming at least the entire outer volution. 
Locality and Position. One hundred and fifty miles above the mouth of Milk riverj 
on the Missouri, in Montana territory; from the Fort Pierre group of the Upper Missouri 
Cretaceous series.” 
Two specimens are found in the present collection, one being only 
15 mm. in diameter and probably far from complete, and the other 11 cm. 
across and possessing more than 6 cm. of living chamber, measured along 
its central axis. This larger specimen is well preserved, and has been 
dissected so as to show surface markings, suture lines, umbilicus, etc., 
of both young and old stages of development. The two flexures of curva- 
ture are well shown, one occurring at the last septum and the other about 
4 cm. back along the periphery. The expansion of the umbilicus is also 
well shown. 
Thus in an undistorted specimen, the characters which Meek noted 
as separating this form from the genus Phylloceras are verified. Crickmay 
concluded after examining this specimen that Rhaeboceras was established 
as a separate genus, which is not related to Phylloceras as Meek supposed, 
but is probably descended from the Hoplitidae. 
Occurrences. In the Baculites compressus zone of the Bearpaw for- 
mation, on the south branch of Box Elder creek, sec. 30, lot 10, range 
29, W. 3rd mer. (Sask.). Collector, W. S. Dyer. 
