ON THE FAUNA AND FLORA OF THE TRIAS OF THE BRITISH ISLES. 299 
in the species to which it belongs. It is, moreover, valuable as exhibiting 
the anterior limits of the angular and splenial bones. 
Various drawings of the fossil, of one-half the natural size, are given 
in the accompanying Plate IT. The articular end of the mandible is lack- 
ing on both sides, but the right ramus is complete so far back as the 
position of the internal vacuity (fig. 3, v.). Each ramus is more or less 
laterally compressed, being twice as deep as broad near the hinder end 
of the tooth-bearing portion (fig. 4), and gradually tapering towards the 
short symphysis, which is comparatively weak and depressed (figs. 1, 5). 
It is hollow at least as far forwards as the fractured end of the fossil on 
the left side. The dentary bone (fig. 2, d.) forms the greater part of the 
outer face of the mandibular ramus so far as preserved, but its precise 
extent in the symphysial region is uncertain. Its outer surface is 
smooth and slightly convex, while its oral border bears a single close 
series of slender conical teeth, which are nearly uniform in size for the 
greater part of its length, but diminish both at the hinder end and at the 
side of the symphysis where they flank the enlarged inner teeth. The 
angular bone (ag.) is shown on the outer face to taper rapidly in front 
beneath the hinder part of the dentary, where it is ornamented with a 
few coarse ridges, which radiate forwards from a point situated postero- 
inferiorly to the piece of fossil preserved. The long and narrow bone (id.) 
which forms the lower portion of the outer face of the mandibular 
ramus in front of the angular just described, may be appropriately 
named infradentary. Its outer face bulges rather more than the 
dentary, and is only faintly marked with stout reticulating ridges 
until it approaches the symphysis, where it seems to spread over the 
flattened lower face of the jaw and its ornamentation becomes con- 
spicuous. This lower face (fig. 5) exhibits the jagged median suture in 
which the two rami of the mandible are firmly united, but it does not 
clearly show any other sutural lines. The upper part of the inner face 
of the mandibular ramus is formed by a long and narrow smooth splenial 
element (fig. 3, sp/.), which terminates in front just before reaching the 
symphysis. During the greater part of its length it rises into a low 
parapet on the inner edge of the rather wide alveolar groove in which 
the teeth are fixed at the outer edge. Below the narrow splenial there 
are two deeper lamin of bone (x, y), separated by a very oblique 
suture, which is inclined downwards and forwards. These bones are 
quite smooth where they form the inner face of the jaw, but exhibit. an 
ornament of stout reticulating ridges where they bend into the lower 
face, and may be continuous respectively with the angular and infra- 
dentary elements already described. When viewed from above (fig. 1) 
and behind (fig. 6) the symphysis distinctly exhibits the jagged median 
suture, but its precise constitution is not clear. It is impressed behind 
by a pair of slit-like hollows (h.), which may have formed the insertion 
for the genio-hyoid muscles. Its flattened upper or oral face is excavated 
on each side by a large shallow oval pit (/.), with the long axis transverse, 
just internal to the row of marginal teeth. In the front part of each pit 
are implanted two relatively large conical teeth which would probably 
be shed alternately. A second pair of diminutive oval pits occurs just 
behind, containing in the fossil two and three small conical teeth 
respectively, Nearly all the teeth are partially obscured by the sandy 
matrix which has necessarily been left to strengthen such fragile pro- 
minences ; but they are seen to be nearly round in section, with a very 
