No. I.] RELATIONSHIPS OF PROTOSTEGA. 228 
greater part of the plastron and limbs of a very large individual. 
The bones are in excellent condition, and the sutures very 
distinct. There are present the hyoplastra of both sides, the 
hypoplastron of the left side, and weathered fragments of that 
of the right, the xiphiplastron of the left side, and the distal end 
of that of the right, the nuchal and eight peripherals determinable 
as belonging in series on the left side, and fragments of others, the 
humeri, scapulae, and coracoids of both sides, and the proximal 
ends of the radius and ulna (?). The femur of the left side, 
the pubis and ilium of the same side with the distal end of the 
ischium, the ischium of the right side with the distal ends of 
the ecto- and ento-pubis. Several incomplete ribs, Pl. IV. 
The second and smaller specimen preserves the humerus, 
scapula, and coracoid of the left side, a singularly complete 
pelvis, and some incomplete ribs. The greater part of the 
skull is in fairly good condition, showing the basi-, supra-, and 
ex-occipitals, the paroccipital, quadrates, petrosals, quadrato- 
jugal, and squamosal, the basisphenoid, pterygoids, and pala- 
tines, and the almost perfect lower jaw. 
Skull. — The supraoccipital was greatly flattened from side 
to side in the process of fossilization. The ridge forming the 
upper edge of the bone slants downward and backward, its dis- 
tal part is incomplete, though apparently only a small part has 
been lost. The superior-anterior portion bears a narrow face 
which slants downward and forward for a considerable distance. 
These regions are almost identical with the same regions in the 
Chelonitdae, and are widely different from Dermochelys, where 
the upper edge of the supraoccipital is almost horizontal, and 
is broad and rounded. The face on the anterior aspect is broad, 
horseshoe-shaped, and almost vertical. 
The region bearing the articular faces for the exoccipitals, 
petrosals, and paroccipitals is moderately expanded and is quite 
solid, showing the absence of any great amount of intercalated 
cartilage, such as occurs in Dermochelys, where the articular 
faces are represented by rugose pits, and are not distinguish- 
able one from the other. The articular faces for the petrosal 
and paroccipital meet on the summit of a ridge running out- 
wardly from the external edge of the anterior semicircular canal. 
