MEGALOSAURUS, Bkld.—The true dinosaurian cha racters of this reptile have been established by the discovery of the sacrum, which consists of five vertebrae, interlocked by the alternating position of neural arch and centrum. The articular surfaces of the free vertebrae are nearly fiat ; the neural arch develops a platform which in the anterior dorsals supports very long and strong spines.
The compressed piercing and trenchant form of tooth which characterises the existing varanian lizards was manifested by the Megalosaurits. The specimen which is most illustra tive of the dental peculiarities of this gigantic reptile is a portion of the lower jaw with a few teeth. The first cha racter which attracts attention in this fossil is the inequality in the height of the outer and inner alveolar walls ; a similar inequality characterizes the jaws of almost all the existing lizards. But in these the oblique groove, so bounded, to which the bases of the developed teeth are anchylosed, is much more shallow, and is relatively wider ; and the teeth in all the stages of growth are completely exposed when the gum has been removed.
In the Megalosawnts the greater relative development of the inner alveolar wall, as compared with the dentigerous part of the jaw in existing Saurians, narrows the dental groove, and covers a greater proportion of the bases of the teeth, besides concealing more or less completely the germs of their suc cessors. Moreover, instead of the mere shallow impressions upon the inner side of the outer alveolar plate to which the teeth are attached in modern lizards, there are distinct sockets formed by bony partitions connecting the outer with the inner alveolar wall in the jaw of the Megalosaurus. These parti tions rise from the outer side of the inner alveolar wall in the form of triangular vertical plates of bone, and from the middle of the outer side of each plate a bony partition crosses to the outer parapet, completing the alveoli of the fully-formed or more advanced teeth ; the series of triangular plates forming a kind of zig-zag buttress along the inner side of those alveoli.
The outer parapet rises an inch higher than the inner one.
Fig. 75 exhibits a portion of another jaw of the Megalo saurus, also from Stonesfield oolite, from which the inner wall has been removed to show the germ of a successional tooth c, about to succeed an old tooth a, which has been broken, and near to which is a newly-formed tooth, b, coming into place. These teeth will exemplify the shape of the crown of the tooth, which is subcompressed, slightly recurved, sharp edged, and sharp-pointed, the edges being minutely serrated ; the edge upon the convex or front border b becomes blunted as it descends about two-thirds of the way towards the base of the tooth ; that upon the concave hinder border a is continued to the base. The lower half of the crown is thicker towards the fore margin than towards the hind one ; so that a transverse section, like that above, a, in fig. 75, gives a narrow oval form pointed behind. At the upper half of the crown the sides slope more equally from the middle thickest part to both margins, and the section is a narrow pointed ellipse. The crown is covered by a smooth and polished enamel, which Section of jaw with teeth of the Megalosaurva Bucklandi, nat. size.
wholly forms the inaTinal serrations. The base of the tooth is coated with a smooth, lighter-coloured cement, forming a thin layer, and becoming a little thicker towards the implanted end of the tooth. The remains of the pulp are converted into osteo-dentine in the basal part of the completely formed tooth. Moderately magnified, the surface of the enamel pre sents a finely wrinkled appearance. The marginal serrations show, under a somewhat higher power, that the points are directed towards the apex of the tooth—a structure well adapted for dividing the tough tissues of the saurian integument.
The main body of the tooth consists of dentine, of that hard unvascular kind of which the same part of the teeth of existing crocodiles and most mammals is composed. No part of the dentine is pervaded by medullary canals, as in the Iguanodon.