KIMMERIDGE CLAY. 5 
versely a broader median convexity, bounded by lateral shallow concavities. On the 
transverse line, between the sixteenth pair of alveoli, are the anterior ends of the palatine 
bones, which are divided by a median suture (PI. I, 20, 20). The major part of the 
palato-nares are bounded by the pterygoids (ib., 24, 24), which extend backward to the 
hase of the occipital condyle (ib., 1), underlapping the basi-sphenoid and_basi-occipital, 
developing ridges which project below the level of the lateral parts of the fossa, and con- 
verge to meet behind the area, including the posterior nostrils. External to these ridges 
the pterygoids diverge to abut against the tympanic pedicles. The mesial border of an 
ectopterygoid is preserved at 25, fig. 1. 
The number of alveoli in each ramus of the mandible (PI. I, fig. 3) is twenty-five or 
twenty-six. The five alveoli corresponding with the premaxillary sockets in the upper 
jaw are the largest. They are separated by similar intervals. Between the fifth and the 
sixth alveolus is a diastema of about 8 lines; the long diameter of the sixth alveolus is 
1 inch 10 lines. An interval of 5 lines divides it from the seventh socket. The succeed- 
ing ones are closer together: they gradually increase in size to the twelfth or thirteenth, 
but do not obtain the size of those opposed to them above; they then gradually decrease 
in size and depth to a diameter of about half an inch. 
The summits of crowns of successional teeth protrude from fosse at the inner and 
back part of the anterior alveoli. The crown of a more advanced successional tooth 
projects into the bottom of the socket of the third and fifth of the symphysial series : 
these teeth show the characters of the genus Pliosaurus. 
The inter-alveolar part of the ‘symphysis mandibule” forms a median longitudinal 
rising, less convex or ridge-like than the one on the palate above. Fossve are discernible 
on the inner side of the mandibular alveoli, but less marked in the upper jaw. The apex 
of a successicnal tooth appears in two of these pits. On the inner side of the posterior 
third of the mandibular ramus there is a wide and deep channel between the surangular 
(29) and angular (30) elements ; and this groove is continued forward indicative of the 
upper border of the splenial (31) which extends along the inner side of the lower half of 
the dentary nearly to the symphysis. The articular surface of the mandible (29), 7 inches 
in transverse, and 5 inches in antero-posterior extent, is slightly concave transversely at 
the inner three fourths of its extent, and then gently convex at the outer fourth; it is 
more concave from before backwards in the major part of its extent, but the peripheral 
boundary is not entire. 
The plesiosaurian affinities, as contradistinguished from the ichthyosaurian, are exemplified 
in the more complete and separate sockets of all the teeth, and in the smaller proportion 
contributed by the premaxillaries to their support and to the formation of the upper jaw. 
The palato-nares of Pliosawrus are more linear and approximate than the species of 
Plesiosaurus (Pl. Hawkinsit, Pl. XVI ;* and Pl. rostratus, Pl. XIII,* ib.), in which 
they have been observed. 
* Vol. of Pal. Soc. for the year 1863; Monogr. of the ‘ Reptilia of the Liassic Formations,’ 1865. 
