286 



and with the expanded extremity nearly circular at the border and 16 lines 

 wide. The upper part of the bone presents a nearly straight slope in its 

 length, and is convex transversely. The under part is likewise straight along 

 the middle, transversely convex posteriorly, and nearly flat at the expanded 

 end. The lower surface of the latter presents near the middle a pair of 

 vascular foramina, and several similar foramina are found near the border. 

 The articular end is transversely -elliptical and barely depressed. Its trans- 

 verse diameter is 15 lines; its vertical diameter 10^ lines. 



Order Sauro])te7-ijgia. 

 OLIGOSIMUS. 



OlIGOSIMUS GRANDiEVUS. 



A fossil ol)tained on Henry's Fork of Grreen River, Wyoming, during 

 Professor Hayden's exploration of 1870, would appear to indicate an extinct 

 reptile allied to Plesiosaurus and Discosaurus. In general aspect, the 

 specimen is different from those in company with it, and I think it doubtful 

 whether it was an associate of the other fossils, which belong to the Bridger 

 Tertiary formation. It was found as a detached specimen, and has no 

 adherent matrix. It probably is of Cretaceous age. 



The fossil, represented of natural size in Figs. 18, 19, Plate XVI, consists 

 of the body of a caudal vertebra, apparently from the root of the tail. It 

 was evidently from a mature animal, as the neural arch was firmly co-ossified, 

 leaving no trace of the original separation. 



In shape and construction the body resembles the corresponding portion 

 of the vertebrae in Plesiosaurus and Discosaurus, but the proportion of length 

 to the other dimensions is much less, and the depth also is not so great. 



The body is biconcave, the concavities being of moderate and nearly equal 

 depth. Deepest at the central half of the area, the peripheral half of the 

 articular surfaces becomes more aljruptly shallower, and with the deflexed 

 edges somewhat convex. Near the border, the articular surfaces are defined 

 by -a narrow circular groove. 



The posterior articular surface of the body at the sides below is deflected 

 in a pair of widely separated facets for a chevron-l>one. The facets are 

 sustained on processes extending forward more than a third of tlie length of 

 the body. Similar facets and processes are absent on the front of the bone. 



