S. W. Williston—Eind Limb of Protostega. 277 



Case cited above. The relationships to Chelone and Thalasso- 

 chelys pointed out by Baur, Hay and Case receive additional 

 confirmation from the structure of the limb, as will be seen in 

 the accompanying figure. The leg, while broader and more 

 powerful, is not essentially different in structure from that of 

 Thalassochelys, and it would seem that there could hardly be 

 longer a question as to the relationship of these forms, — 

 Protostega and Archelon, at least, — to the Cheloniidae. 



The characters separating Archelon Wieland from Pro- 

 tostega Cope, while not very important, would seem sufficient. 

 Nevertheless, one can derive little justification from the dif- 

 rent geological horizons in which the forms are found. The 

 relations between the Niobrara and Fort Pierre vertebrates are 

 for the most part very close. I have recognized in both hori- 

 zons Tylosaurus, Plaieoarpus and Mosasaurus (Clidastes), 

 as well as Pteranodon and Hesperomis, all very typical of the 

 Niobrara deposits, and the existence of Claosaurus has been 

 recently affirmed in the Fort Pierre. On lithological grounds, 

 there is nothing separating the two groups of deposits, and 

 I protest against the names Colorado and Montana, as per- 

 petuating a wrong impression. On paleontological and litho- 

 logical grounds there would be much better reasons for uniting 

 the Niobrara with the Fort Pierre than with the Fort Benton. 



Description. — The head of the femur is large, and, in life, 

 evidently nearly hemispherical. The neck is very stout, 

 placed at nearly right angles to the axis of the shaft and is but 

 slightly constricted. The trochanter is large, and stout, with 

 a large, triangular, roughened area on the posterior side for 

 muscular attachment. The smaller trochanter is indicated by a 

 small tuberosity. The shaft is much constricted and curved, 

 with its convexity dorsal ; it is nearly cylindrical at its middle 

 part. The condyles are large and stout, the inner more mas- 

 sive than the outer one; their articular surface looks nearly 

 backward. The tibia is much expanded superiorly, and has its 

 articular surface at an angle of about 45° with the axis of the 

 shaft. On its posterior surface, and margin, a little below the 

 angle there is a strong muscular rugosity. The shaft is much 

 narrowed below, and is again moderately expanded for the 

 distal articulation. 



The fibula is elongated and narrow, of nearly uniform width, 

 except at the upper extremity. This portion of the bone 

 is wanting in the specimen but that portion preserved 

 indicates a moderate expansion superiorly. On the posterior 

 surface, opposite the roughening of the tibia, there is a strong 

 rugosity, produced into an angular tubercle, for muscular 

 attachment. 



Three tarsal bones are preserved, and there was probably 



Au. Jour. Sci. — Fourth Series, Yol. XIII, No. 76.— April, 1802. 

 19 



