THE CARAPACE AND PLASTRON OF BASILEMYS SINUOSUS, 



A NEW FOSSIL TORTOISE FROM THE LARAMIE 



BEDS OF MONTANA. 



BY E. S. RIGGS. 



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 filially by Hay to a proposed new genus, Basilemys% 

 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 sinuosus 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. 957. 

 fThe Ottawa Naturalist, Vol. XVI. p. 63. 

 tNomen Nudem, Bull. U. S. Geol. Surv. Nov. 179, p. 445. 



249 • 



