THECODONTIA. 277 



sinks iuto 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 vertebras of the Rhyncliosaurus from the new red 

 sandstone of Shropshire. 



Besides deviating from existing lizards in the thecodont 

 dentition and biconcave vertebrae, 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 coracoid. The sacrum includes 

 at least three vertebrae. The humerus appears to have been 

 little more than half the length of the femur, and to have been, 

 like that of the Rhyncliosaurus, 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 

 shews 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 slight one anteriorly. 



The tibia, fibula, and metatarsal bones manifest, like the 

 femur, the fitness of the Saurians 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 Thccodonto- 

 scurus and Palceosaurus ; 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- 



