THE CARAPACE AND PLASTRON OF BASILEMYS SINUOSUS, 
A NEW FOSSIL TORTOISE FROM THE LARAMIE 
BEDS OF MONTANA. 
BYGESSeRIGGS, 
An unusually well-preserved-shell of a tortoise (P. 12008) col- 
lected by the Museum Expedition of 1904 presents new and inter- 
esting characters. The specimen is evidently closely related to a form 
described* by Cope from the Judith River Beds of Montana as Comp- 
semys variolosus. This, with a more complete specimen from the 
Belly River series of Alberta, was later referred} by Lambe to the 
genus Adocus, and finally by Hay to a proposed new genus, Basilemyst 
The size of the specimen under consideration is almost identical 
with that described by Lambe and the dermal markings are very 
similar, but it differs notably in the form of the anterior end of the 
plastron and the arrangement of gular shields. The median sulcus 
of the plastron is remarkably sinuous, crossing and recrossing the 
median line and often deviating widely from it. On account of this 
character the specific name stmuosus is proposed for it. It is provi- 
sionally referred to Basilemys pending the definition of that genus. 
_The specific characters are as follows: Anterior end of plastron 
produced into a thick rounded knob; gular shields long and narrow 
and meet at the median line; median sulcus unusually sinuous. 
The specimen as a whole is in a fine state of preservation. It 
was found in a bed of hard but uncemented sand containing bones 
of Triceratops, Trachodon, and other characteristic Laramie fossils. 
The shell was lying on the plastron in its normal position. It has 
been somewhat flattened by compressure in the vertical direction. 
The carapace is complete with the exception of a few fragments miss- 
ing from the vertebral region and a section from the right side, in- 
cluding the posterior half of the second and the anterior half of the 
third pleural plates, together with the greater part of the fifth and 
the anterior part of the sixth peripheral plates. Much of the con- 
tact between the carapace and plastron is also lost. The remainder 
of the plastron is preserved entire. However, the inferior surface 
is modified by compressure so as to be deeply concave. 
*Pro. Acad. Nat. Sci. Phila. 1876, p. 257. 
+The Ottawa Naturalist, Vol. XVI. p. 63. 
tNomen Nudem, Bull. U.S. Geol. Surv. Nov. 179, p.445. 
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