Page:Quarterly Journal of the Geological Society of London, vol. 26.djvu/130

44 restrict the name Palæosaurus to the latter (or Megalosauroid) form of tooth, and to use Thecodontosaurus for the former (or Scelidosauroid) type, the varieties of which may be embraced under the common name of platyodon.

The bones referred to and described by Riley and Stutchbury are vertebræ, ribs of two kinds, a clavicle, two "coracoids," a humerus, a "radius," two femora, an "ischium," a tibia, a fibula, metacarpal and metatarsal bones, and ungual phalanges.

The "coracoid" figured is, as I suspected, a fragmentary ilium. The "radius" I take to be a tibia. The parts of the skeleton which diagnose the Dinosaurian nature of these reptiles, in addition to the teeth, are:—1, a caudal vertebra with the chevron bone; 2, an ilium; 3, a tibia.

The diagnostic mark in the first part of the skeleton mentioned lies in the complete union of the crura of the chevron bones at their proximal ends, in consequence of which coalescence the fork of the chevron bone is converted into a foramen (Pl. III. figs. 5 & 6). This character appears to be universal among the Dinosauria.

With respect to the ilium (Pl. III. fig. 7), it has every character of that bone in the Dinosauria. That part which enters into the acetabulum forms a semicircular arch, the piers of which are formed by the præ- and postacetabular processes (a, b), both of which are strong and trihedral. They are about equal in length; and each ends in a truncated face, which looks a little downwards and a little forwards in the anterior, downwards and a little backwards in the posterior process. The expanded supraacetabular part of the ilium (c) is a vertically disposed plate, equal in height to the acetabular part. Anteriorly (d) it is produced in front of the acetabulum for a length equal to that of the neck by which it joins the acetabular part. Posteriorly (e) it is prolonged into only a very short process, which does not project as far backwards as the postacetabular apophysis.

In all these respects the Thecodontosaurian ilium exaggerates the peculiarities of that of Megalosaurus. And the like is true of the form of the outer and inner surfaces, and of the superior contour, of the supraacetabular part. In the Triassic Dinosaurian the outer surface of this part of the bone is strongly concave from before backwards above the acetabulum, the posterior iliac process being sharply bent outwards; while it becomes flat above the anterior iliac process. Hence the superior contour has a sort of ∫-like curve. The supraacetabular part of the ilium of Megalosaurus has the same curvature, though less strongly pronounced. The inferior surface of the anterior process of the supraacetabular part of the ilium of Megalosaurus presents a narrow groove, bounded on each side by ridges of bone. In the Thecodont, the place of the groove is taken by a broad surface which is only slightly concave from side to side. In Megalosaurus the posterior iliac process is a little longer than the postacetabular, and possesses a considerable height. In the Thecodont it is shorter, and much lower and more tapering posteriorly.