most distal is much smaller and lies about half-way between the former and the tip of the tooth. The anterior edge presents no such denticulations. It is quite smooth (but amongst the loose teeth a somewhat different arrangement is met with, as will be shown later).

The “root” is broken across at the base. It is covered with a pale buff cementum, and is loosely embedded in the alveolar groove, the floor of which it nearly reaches. At the broken end it is nearly oval in section, but rather broader at one end of the oval. There is a small pulp cavity traversing the black dentine (Fig. 1a). I must emphasise the absence of sockets in this portion of the jaw, for in the other portion they are distinct. Nor is there any furrow on the outer surface to indicate the presence of more than one bone. In transverse section this portion of the jaw which, as I shall show, is more anterior than the other portion (b), is V-shaped (Fig. 2);

Fig. 1.—A tooth from the jaw-fragment “a” after removal of the matrix. 1a.—View of the broken end of this tooth, showing the pulp-cavity.
Fig. 2.—The end of the jaw-bone “a” viewed in perspective, showing the tooth in situ.
Fig. 3.—Crowns of two isolated and broken teeth.

one limb, which I regard as external, is taller and stouter than the other and somewhat thickened along the upper half, whereas the internal limb of the V is thin and tapers slightly upwards.

Amongst the isolated teeth there is one with the crown entire; this has a series of seven larger and smaller denticulations along one edge, which are spaced, and there are none of the finer ones at the base (Fig. 3). Another crown shows denticulations on both the anterior and posterior margins; those on one edge, probably the posterior, are larger and fewer than on the other edge. Unfortunately this crown is incomplete.

The other fragment of the jaw (b) consists of two mandibles lying close together, with their inner surfaces facing, and are evidently in their natural position. One, that of the right side, is

rather longer than the other, as the fracture at one end has not gone right across the two. The longer, or right mandible (Fig. 4) measures 12 cm. in length. Its height at one end (anterior) is 3.5 cm., and at the other end is 4 cm. The shorter of the two mandibles (Fig. 5) is 9 cm. in length and 3.5 cm. in height. The outer surface of the mandible is traversed by a narrow, shallow furrow which passes obliquely downwards and backwards, starting at a distance of 10 cm. from upper edge anteriorly, to a point 23 cm. below at the posterior end. In transverse section an extremely fine line can be traced inwards from this furrow, so that the jaw seems to be composed of two bones closely in contact (see later). Fortunately, one end, the posterior, of the fragment shows a clean fracture across the two mandibles and gives a picture precisely like the figure of a transverse section of the pair of mandibles of Ichthyosaurus. The thickness of the single mandible is 3.3 cm.

Fig. 4.—Side view of the right mandible of fragment “b” showing the external furrow separating the two constituent bones of the jaw. A, the anterior end.

It will be noticed in the figure that the right and left alveolar grooves face outwards, away from one another; yet the association of the two mandibles must be natural, for no accidental association could, I imagine, produce so symmetrical a figure.

The upper surface of the jaw presents a deep alveolar groove whose outer wall is thinner and of less height than the inner, and the latter is traversed by a small circular vascular foramen (the bones show up very distinctly when the cut surface is wetted, for they appear nearly black against the grey matrix). The longer, that is, the right jaw, supports the base of a broken tooth, situated about 4 cm. from the posterior end; it projects about 1.8 cm. and has a diameter of 1.2 cm. The root is embedded in the bone. Its long axis is oblique, the broken crown being directed towards the anterior end of the jaw. This tooth is solid, for the pulp cavity does not extend upward so far as the crown, for the crown is not preserved—

it is only the root that remains. The dentine is black, covered with cementum of buff colour, rather lighter than the colour of the bone.

The shorter (left) mandible also contains the base of a broken tooth (it is possible that some of the loose teeth belong to this jaw). The fracture is evidently recent and perhaps was made while Thomson was excavating the bone from the matrix. This tooth is at a distance of 5.5 cm. from the posterior end, and thus corresponds with that of the right jaw. At the anterior broken end is the broken root of another tooth, oval in section and surrounded by bone, and covered above by a bony mass which blocks up the alveolar groove

Fig. 5.—Side view of the shorter, left mandible of fragment “b.” The upper edge of the bone had been broken away, hence the less height of this as compared with the right mandible. The tooth has been broken short, and the lower edge chipped showing matrix.

Fig. 6.—Upper surface of the left mandible, showing the curious septal tissue crossing and filling the alveolar groove before and behind the base of the tooth. The alveolar groove at posterior end has been freed from the matrix.

(Fig. 7). When the matrix was scraped away so as to explore this groove the fact was revealed that the groove is not continuous, but is blocked at intervals by irregular masses of bone which extend apparently from the inner surface outwards, and in one case ends in a claw-like ridge on the upper surface of the outer wall of the jaw-bone (Fig. 6). These “sockets” are thus very irregular in shape and extent, and in the degree to which the septa are developed on the two mandibles, for on the right one they are but feebly developed, while on the left they are very stout. The appearance presents some similarity to those figured by Owen for Megalosaurus bucklandi, with which, however, this fossil has no affinity. ^*