Sp. Thecodontosaurus antiquus.—In 1836 certain reptilian remains from the "dolomitic conglomerate" at Redland, near Bristol, were described by Messrs. Riley and The matrix has been referred to the Permian period ; it is now thought by some good observers to be not older than the t riassic.
The teeth in these reptilian fossils are lodged in distinct sockets ; they are arranged in a close-set series, slightly de creasing in size towards the posterior part of the jaw ; each ramus of the lower jaw contained twenty-one teeth. These • Geological Transactions, 2d series, vol. v., p. 344.
are conical, rather slender, compressed and acutely pointed, with an anterior and posterior finely-serrated edge, the serra tures being directed towards the apex of the tooth ; the outer surface is more convex than the inner one; the apex is slightly recurved ; the base of the crown contracts a little to form the fang, which is subcylindrical.
Genus PALEOSAURUS, Riley and Stutchbury. In the same formation as contained the jaw and teeth of the Thecodontosaw rus two other teeth were separately discovered, differing from the preceding and from each other ; the crown of one of these teeth measuring nine lines in length and five lines in breadth. It is compressed, pointed with opposite trenchant and serrated margins, but its breadth as compared with its length is so much greater than in the Theeodontosaurus, that upon it has been founded the genus Palccosaurus, and it is distinguished by the specific name of platyodon, from the second tooth, which is referred to the same genus under the name of Palceosaurus cylindrodon. The portion of the tooth of the Palceosaurus cylindrodon which has been preserved, shows that the crown is subcompressed and traversed by two opposite finely-serrated ridges, as in the Thecodantosaurus ; its length is five lines, its breadth at the base two lines.
The vertebrae associated with the two kinds of teeth above described are sub-biconcave, with the middle of the body more constricted, and terminal articular cavities rather deeper than in Teleosaurus ; but they are chiefly remarkable for the depth of the spinal canal at the middle of each vertebra, where it sinks into the substance of the centrum ; thus the canal is wider, vertically, at the middle than at the two ends of the vertebra : an analogous structure, but less marked, obtains in the dorsal vertebrae of the laynchosaurus from the new red sandstone of Shropshire.
Besides deviating from existing lizards in the thecodont dentition and biconcave vertebra-, the Saurians of the dolomitic conglomerate also differ in having some of their ribs articulated by a head and tubercle to two surfaces of the vertebra, as at the anterior part of the chest in crocodiles and Dinosaurs. The shaft of the rib was traversed, as in the Protorosaur and Rhynchosaur, by a deep longitudinal groove. Some fragmen tary bones indicate obscurely that the pectoral arch deviated from the crocodilian, and approached the lacertian or enalio saurian type, in the presence of a clavicle, and in the breadth and complicated form of the corticoid. The sacrum includes at least three vertebra. The humerus appears to have been little more than half the length of the femur, and to have been, like that of the Rhynchosaurus, unusually expanded at the two extremities. The femur is chiefly remarkable for a third process or trochanter, just above the middle of the shaft, which shows a medullary cavity. The distal condyles are flattened, the outer one being the larger ; there is a deep depression between them posteriorly, and a very light one anteriorly.
The tibia, fibula, and metatarsal bones manifest, like the femur, the fitness of the Saurian for progression on land. The ungual phalanges are sub-compressed, curved downwards, pointed, and impressed on each side with the usual curved canal.
The following conclusions may be drawn from the know ledge at present possessed of the osteology of the Thscodonto saurus and Palaosaurus : in their thecodont type of dentition, biconcave vertebrae, double-jointed ribs, and proportionate size of the bones of the extremities, they agree with the amphi ccelian crocodiles ; but they combine a dinosaurian femur, a lacertian form of tooth, and a lacertian structure of the pec toral and probably pelvic arch with these crocodilian charac ters ; and they have distinctive modifications, such as the moniliform spinal canal, in which, however, the almost con temporary Rhynchosaur participates. It would be interesting to ascertain whether the caudal vertebrae are characterized, as in the Thuringian Protorosaur, by double diverging spinous processes.