Ch. xxiii.] thecodont saurians. 355 



spot of the debris of those rocks on which it immediately rests. At 

 one point we find pieces of coal-shale, in another of mountain lime- 

 stone, recognizable by its peculiar shells and zoophites. Fractured bones, 

 also, arid teeth of saurians, are dispersed through some parts of the 

 breccia. 



These saurians (which until the discovery of the Archegosaurus 

 in the coal were the most ancient examples of fossil reptiles) are all 

 distinguished by having the teeth implanted deeply in the jaw-bone, 

 and in distinct sockets, instead of being soldered, as in frogs, to a simple 

 alveolar parapet. In the dolomitic conglomerate near Bristol, the re- 

 mains of species of two genera have been found, called Thecodontosaurus 

 and Palceosaurus by Dr. Riley and Mr. Stutchbury ;* the teeth of which 

 are conical, compressed, and with finely serrated edges (figs. 459 and 

 460). 



Teeth of Saurians. Dolomitic conglomerate ; Kedland, near Bristol. 

 Fig. 459. 460. 



Tooth of Palceosaurus Mil » Tooth of Thecodontosaurus. 



% platyodon, nat. size. iff 1\ 3 times magnified. 



Sir Henry de la Beche has shown that, in consequence of the isolated 

 position of the breccia containing these fossils, it is very difficult to de- 

 termine to what precise part of the Poikilitic series they belong.f Some 

 observers suspect them to be triassic ; but, until the evidence in support 

 of that view is more conclusive, we may continue to hold the opinion of 

 their original discoverers. 



In Russia, also, Thecodont saurians of several genera occur, in beds of 

 the Permian age ; while others, named Protorosaurus, are met with in 

 the Zechstein of Thuringia. This family of reptiles is allied to the living 

 monitor, and its appearance in a primary or paleozoic formation, observes 

 Prof. Owen, is opposed to the doctrine of the progressive development of 

 reptiles from fish, or from simpler to more complex forms ; for, if they 

 existed at the present day, these monitors would take rank at the head of 

 the Lacertian order.J 



We learn from the writings of Sir R. Murchison,§ that in Russia 

 the Permian rocks are composed of white limestone, with gypsum and 

 white salt ; and of red and green grits, occasionally with copper ore ; also 

 magnesian limestones, marlstones, and conglomerates. 



* Geol. Trans., Second Series, vol. v. p. 349, pi. 29, figures 2 and 5. 



f Memoirs of Geol. Survey of Great Britain, vol. i. p. 268. 



% Owen, Report on Reptiles, British Assoc, Eleventh Meeting, 1841, p. 197. 



§ Russia and the Ural Mountains, 1845 ; and Siluria, ch. xii. 1854. 



