24 CASE. [Vou. XIV. 
This canal is represented by a deep triangular pit, no part of 
which is covered by processes from the sides. 
This condition of the semicircular canal is exactly that of the 
Chelontidae, and differs widely from Dermochelys, in which it is 
roofed by three distinct processes meeting in the middle and 
leaving three foramina for communication with the other semi- 
circular canals (Pl. V, Fig. 1). 
The exoccipital of the left side is badly crushed, but is still 
in sutural union with the paroccipital of the same side, and the 
two are in connection with the supraoccipital. The exoccipital 
of the right side is separate and almost perfect. The ascending 
process for the supraoccipital is short and strong. The descend- 
ing process is short, and did not reach connection with the 
pterygoid. The articular face for the paroccipital is deeply 
excavated. The condylar foramen is near the condylar portion, 
which is well ossified and free from osseous connection with 
the same region of the basioccipital. This complete and sepa- 
rate ossification of the condylar region is a point of decided 
difference between the Chelontzdae and Dermochelys; in the 
latter the region is almost entirely cartilaginous, and the three 
bones are weakly anchylosed in old specimens (PI. V, Fig. 2). 
The Jdasioccipital is a comparatively broad and short bone 
with well-ossified condylar portion and strong lateral processes 
terminating in rugose extremities which extended between the 
pterygoids and the exoccipitals. The under surface is nearly 
smooth, and lies in the plane of the horizontal axis of the skull. 
The articulation for the basisphenoid is confined to its anterior 
end. 
In every particular but that of the ossified condylar portion 
the basioccipital of Protostega agrees with Dermochelys. In the 
Cheloniidae the lateral processes are small, and the pterygoid 
articulates with the exoccipital; this causes the basioccipital to 
lie largely between the exoccipitals, instead of below them as 
in Dermochelys. The inferior surface of the basioccipital in 
the Cheloniidae varies from being almost horizontal to being 
inclined steeply downward and forward, and the basisphenoid 
may cover it far back towards its middle (Pl. V, Fig. 3; a, from 
above; 4, from below). 
