No. I.] RELATIONSHIPS OF PROTOSTEGA. 35 
supports the neuropophyses. The large centrum exhibits the 
superior groove and antero-lateral platform for support of the 
neural arch. One end is cupped obliquely, while the other is 
nearly plane, with the same obliquity and a slightly raised 
margin. Its outline is subtriangular. The lower side of this 
centrum possesses a short keel posteriorly. The sides exhibit 
no pit, but have a thin edge, which is concave behind the middle 
and then turned outward. I can see no articulation for a rib.” 
These vertebrae are stated by Cope to be most closely 
related to Dermochelys. Unfortunately, the material is too 
limited to admit any positive conclusions to be drawn as to the 
relationship of Protostega ; but it is necessary here to note the 
close resemblance between the cervical vertebrae of Dermo- 
chelys and the Chelonitdae. Both have the strong articular 
process for the nuchal plate on the last cervical, and the articular 
faces between the 6 and 7 are plain. 
Limbs.— The humerus is very broad and strong. The area: 
for cartilaginous attachment on the mesial process is entirely 
separate from the area on the head, which is in turn separate 
from the radial process. In the smaller specimen the areas are 
all united. This is evidently a variation due to age, as the 
same thing is observable in large and small specimens of 
Chelonia mydas. The radial process lies near the center of the 
shaft, and is very prominent. It is simple, instead of having 
the U or V shape of the same process in existing sea turtles 
and in Psephophorus. The ento- and ecto-condyles and the 
entepicondylar and ectepicondylar processes are strong and 
prominent. The ectepicondylar foramen is quite large. The 
shaft of the bone was somewhat flattened and constricted 
beneath the head (Pl. VI, Fig. 12). 
MEASUREMENTS. 
Length from distal end to top of head. . . . . . . . .348 meters 
Greatest width at distal end . . . . AL es OGD (Se 
Length from exterior edge of head to end of mesial process .175 “ 
The humerus shows a somewhat close resemblance to that of 
Psephophorus and Dermochelys. The radial process is simple, 
stands higher on the shaft, and lacks the downward prolonga- 
