250 
PROFESSOR OWEN ON A LARGE EXTINCT LIZARD 
black colour shown in parts of such fossils, that also we find in some Mammalian fossils 
from our own tertiaries.* 
The bases of both teeth in the pleistocene fossil (Plate 12, figs. 2-5, b, b') were 
anchylosed to the alveolar floor continued from the outer wall a, and to this also was 
similarly anchylosed so much of the supposed part of the crown as remained of each 
tooth. The portion of the jaw-bone continued from the inner surface of the anchylosed 
teeth showed a natural surface sloping away from the teeth upon so much as remained 
of the inner surface, a , of the dentary bone. 
Here, therefore, were plain characters, not of a Crocodilian, but of a Lacertian 
mandible, and of a species of that division of the Lacertilia called “ pleurodont.”! 
Of existing Australian Lizards Chlamydosaurus is “ acrodont; Hydrosaurus is 
“pleurodontand, moreover, is the largest known existing Lacertian. The base of 
the tooth in this species is striated, and that character is best shown on the inner side 
(Plate 12, fig. 8), which is free from the bony parapet, according to the fashion 
exemplified in Notiosaurus (Plate 12, fig. 2) ; but with fewer and larger ridges. 
I append figures, nat. size, of a portion of the jaw of Hydrosaurus gigas (Plate 12, figs. 
7, 8), corresponding to the fossil. The proportions of the outer wall, and of the base of 
the teeth thereto anchylosed, are the same ; such confluent part is, also, longitudinally 
ridged. The pleurodont character prevails in both upper and under jaws, but the 
teeth are mostly wider apart in the mandible, and are juxtaposed as in the fossil, only 
in a small proportion of the dentigerous part. At this stage of the comparison a 
vertical transverse section was taken of that end of the fossil to which the more frag¬ 
mentary tooth was attached. This section (ib., fig. 5) demonstrated the anchylosis of 
tooth to bone according to the pleurodont type. A slice of the section was prepared 
for microscopic scrutiny. Under a magnifying power of 120 the coarse lamellate 
disposition of the osseous tissue of the Lacertian mandible, the elongate bone-cells, and 
the fine plasmatic tubules, diverging from the vascular cells, were demonstrated at a, 
fig 9. The basally attached portion of tooth showed the Lacertain vascularity of the 
part and the dentinal tubes radiating from the vascular canals, also the lamellate 
walls of the canals (ib., b). 
Another character was brought to light by this section. The remains of the pulp- 
cavity were seen, on first inspection of the fossil, in an aperture of 2 millims. diameter 
at the middle of the fractured surface of each tooth-crown, fig. 3, c, c. On the 
Crocodilian hypothesis such aperture should expose a pulp-cavity widening as it receded 
from the enamelled crown. In the section above described such cavity or continuation 
of the aperture was longitudinally traversed, and demonstrated its contracting to a 
termination at 6 millims. above the anchylosed base of the tooth (Plate 12, fig. 5, c). 
In Hydrosaurus the outer surface of the dentigerous part of the mandible is per¬ 
forated by neuro-vascular apertures almost as numerous as the teeth, and about the 
* ‘ History of British Fossil Mammals,’ 8vo., 1846, pp. 301, 414, 420. 
f ‘Odontography,’ 8to., 1845, p. 240, plates 67 (Monitor), 68 (Iguana). J Ib., p. 241. 
