No:t.] RELATIONSHIPS OF PROTOSTEGA. 25 
The petrosals are both present in a very perfect condition. 
They are roughly triangular, and have a strong ridge, partly due 
to pressure, on the external surface. The external face also 
shows a deep excavation corresponding to a similar excavation 
on the antero-interior portion of the quadrate. The union of 
the sides of these two excavations forms the foramen for the 
external carotid artery, and probably excluded the paroccipital 
from any part of the foramen. 
The external semicircular canal is represented by a deep pit 
bridged in its antero-superior region by a bony bar reaching 
from side to side, and leaving in front of it a foramen for 
communication with the anterior canal. 
The articular face for the basisphenoid is broad and strong. 
The formation of the carotid foramen, as well as the nature 
of the semicircular canal, is typically that of the Chelonizdae. 
In Dermochelys the paroccipital takes large part in the forma- 
tion of the foramen, and the pit in the petrosal is entirely free 
from any bony processes (Pl. V, Fig. 4; a, from within; 4, from 
without). 
The paroccipital of the right side is present in almost perfect 
condition. The bone is elongated and reaches connection 
with the squamosal, a character which never appears in Dermo- 
chelys. The posterior or external half of the posterior semi- 
circular canal is roofed by a bony process pierced by two 
foramina which communicate with the other canals. This 
character of the posterior semicircular canal appears in the 
Chelontidae, and is very different from Dermochelys, where there 
is a single bony process which does not reach entirely across 
the canal (Pl..V;, Fig: 's): 
The basisphenoid is badly crushed, but retains somewhat its 
original character. It is almost round in outline, with a smooth 
under surface. The upper face is traversed in a longitudinal 
direction by two deep grooves. There is no trace of an ante- 
riorly extending rostrum on the thickened anterior end. The 
smooth under surface appearing largely on the base of the 
skull, with no trace of a ridge where it meets the basioccipital, 
is similar to the condition found in Dermochelys, though in 
that genus the basisphenoid takes much larger part in the 
