KIMMERIDGE CLAY. 7 
Species—PLIOSAURUS TROCHANTERIUS, Owen. Plate III. 
PLusIOSAURUS TROCHANTERIUS, Ow. Report on British Fossil Reptiles, 8vo, p. 85, 
1839. 
In the work above cited the specific character of the fossil Reptile in question was 
indicated by modifications of the femur; but the chief distinction between Pliosaurus 
trochanterius and Pl. grandis is conspicuous in the greater relative extent of the symphysis 
mandibulz in the former, and in the greater proportion of the dental series lodged in that 
part of the lower jaw. This character is exemplified in the fourth admeasurement in the 
“Table,” p. 3, and in PI. II, fig. 3, as compared with Pl. I, fig. 3. 
The surangular developes in Phosaurus trochanterius [P\. I, fig. 4, 29'] a low but 
well-marked angular coronoid process. Anterior to this the upper border of the mandible 
becomes thick and transversely convex; and, an inch below the border, the outer side of 
the ramus is impressed by a wide and deep longitudinal groove. So much of the articular 
surface as is preserved agrees in structure and form with that in Phosaurus grandis ; and 
the extent of the angular projection behind the articular cavity to the same. 
The fore part of the symphysis, including the first three pairs of teeth, has been 
subject to such violent horizontal force as to be crushed in that direction, and broken 
across both the upper and the under surfaces of the rest of the mandible, without having 
been detached from the intervening structure or tissue of the bone. The bottoms of the 
sockets only of the included teeth are preserved, with parts of the partitions which, here, 
are only from 2 to 3 lines thick. ‘These sockets increase in size to the third. The 
diameter of the outlet of the fifth socket, which is the first entire one, measures 1 inch 
9 lines across ; it is rather less longitudinally. ‘The outlets of most of the alveoli are 
subcircular, with a tendency to a subquadrate section, with intervals not exceeding 
2 lines, and they retain a uniformity of size to within four or five sockets at the end of 
the series, which progressively decrease in size. 
The total number of teeth, as shown by sockets, in each mandibular ramus is four- 
teen ; of which ten occupy the symphysial part of the jaw (PI. III, fig. 3). 
The upper surface of the symphysis between the first six teeth is flush with the 
alveolar outlets, is smooth, and slightly convex transversely. Beyond the sixth pair of 
teeth the intervening surface rises above the inner borders of the alveoli as high as half 
an inch between the ninth—eleventh pairs of sockets ; the upper surface of the hinder 
part of the symphysis becomes slightly convex transversely, and the pointed anterior ends 
of the splenials (31) enter into its composition.* 
No part of the upper jaws of this skull of P/iosawrus trochanterius has been preserved ; 
* Some of the matrix retained in the interval appears to prolong, in the figure, the symphysis beyond 
its true posterior limit. 
