384 CRETACEOUS REPTILES. 
terminal articulations of the centrum of the dorsal vertebrae of Mosasaurus gracilis 
present a full oval (not elliptical) form, the long axis of which is vertical and 
the great end downwards (Pl. XX XVII. fig. 4). The length of the centrum (ib. 
fig. 3), which is 3} centimeters, or 1 inch 5 lines, exceeds the breadth ; but this 
is equalled by the height of the centrum. The diapophyses in fig. 2, d, are broken 
away ; in fig. 3 it is uncertain whether the surface be a fractured one, or whether 
it is a natural articular cavity for the rib; the analogy of Mosasaurus Hofmanni 
favours the former view of it. The neural arch (fig. 3, n) is anchylosed to the 
centrum, as in the larger species of Mosasaurus. I can perceive only a feeble 
indication of zygapophyses, which shows that the vertebre (figs. 2, 3 & 4) come 
from the posterior region of the back. The neural canal (fig. 4, 2) is small and 
triangular: a sharp longitudinal ridge rises from the middle of its floor, and on 
each side of this there is a vascular canal descending vertically into the sub- 
stance of the centrum: this substance presents a coarse fibro-cancellous texture ; 
the areole extended longitudinally, and decreasing much in size at the ends of 
the centrum. The outer surface of the vertebra is smooth: the margins of the 
anterior articular concavity are sharp. 
The vertebra (PI. XX XVII. fig. 2) shows, by the lower position of the diapo- 
physis (d), that it comes from a more posterior position of the spine than that 
represented in fig.3. Figs. 4 & 5 give two views of the same vertebra, but ina 
reversed position, the hemal canal h being upwards ; they demonstrate another 
Mosasaurian character in the anchylosis of the hemapophyses or chevron-bones 
to the centrum, as in the posterior caudal vertebra of Mosasaurus Hofmanni : 
but the hemal canal (fig. 4, 2) is relatively wider, and the entire centrum is 
much longer than in the corresponding kind of vertebra figured by Cuvier*. 
Three views of the body of a vertebra of the Mosasaurus gracilis, discovered 
by the Rev. H. Hooper, M.A., distinguished by his geological researches in the 
neighbourhood of Lewes and Brighton, are given in Tab. XXXIX. figs. 5, 6, & 7. 
This specimen is from the Sotheram Pit near Lewes. In the rich collection of 
Mrs. Smith are preserved some of the bones of a fin or paddle of the Mosasaurus. 
From the genus Leiodon+ the Mosasaurus gracilis difters, like the Mosasaurus 
Hofmanni, in the inequality of the two sides of the crown of the teeth, which are 
bounded or divided by the anterior and posterior ridges. The Mosasaurus Maai- 
miliani differs from the genus Leiodon in the polygonal character of the crowns 
of the teeth. 
* Loc. cit. pl. 19. fig. 6, A, B. 7 + Odontography, 4to, p. 261, pl. 72. figs. 1 & 2. 
