270 PALAEONTOLOGY. 



The matrix has been referred to the Permian period ; but it is 

 now thought by some good observers to be not older than the 

 triassic. 



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 

 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 Pal/EOSAUEUS, R. and S. In the same formation as 

 contained the jaw and teeth of the Thccodontosaurus two other 

 teeth were separately discovered, differing from the preceding 

 and from each other ; the crown of one of these teeth measur- 

 ing nine lines in length and five lines in breadth. It is com- 

 pressed, pointed with opposite trenchant and serrated margins, 

 but its breadth as compared with its length is so much greater 

 than in the Thccodontosaurus, that upon it has been founded 

 the genus Palccosaurus, and it is distinguished by the specific 

 name of jrfatyodon, from the second tooth, which is referred to 

 the same genus under the name of Palccosaurus cylindrodon. 

 The portion of the tooth of the Pcdceosaurus cylindrodon which 

 has been preserved, shews that the crown is subcompressed 

 and traversed by two opposite finely-serrated ridges, as in the 

 Thccodontosaurus; 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 



