ON BRITISH FOSSIL, UEPTILES. 153 



middle of the shaft Is 2-^ lines ; the siuface of the preserved middle part shows 

 tlie shaft to have been somewhat angular ; the compact outer wall of the bone 

 is about a quarter of a line thick ; a large medullarj^ cavity extends the whole 

 length of the sliaft, agreeing with the indications of terrestrial habits yielded 

 by the bones before described ; the extremities of the femora are spongy, but 

 nuich deconijjosed and stained with iron-mould. 



There are few genera of extinct reptiles of M'hich it is more desirable to 

 obtain the means of determining the precise modifications of the locomotive 

 extremities than the lilajnchosaurus. The fortunate preservation of the skull 

 has brought to light modifications of the Lacertine structure leading towards 

 Ciwlonia and Birds, which before were unknown ; the vertebra likewise ex- 

 hibit very interesting deviations from the Lacertian type. The entire recon- 

 struction of the skeleton of the Ehynchosaurus may be ultimately accom- 

 plished, if the same interest in the collection and preservation of the fossils 

 of the Grinsill quarries be continued, as has already produced so important 

 an accession to Palajontology through the well-directed zeal of Dr. Ogier 

 Ward and other members of the Literary and Scientific Association at 

 Shrewsbury. 



Thecodonts. 



Among the inferior or squamate Saurians there are two leading modifica- 

 tions in the mode of attachment of the teeth, the base of which may be either 

 anchylosed to the summit of an alveolar ridge, or to the bottom of an alveolar 

 groove, and supported by its lateral wall. These modifications are indicated 

 respectively by the terms " acrodont" and " pleurodont." A third mode of 

 fixation is presented by some extinct Saurians, which, in other parts of their 

 organization, adhere to the squamate or Lacertine division of the order, the 

 teeth being implanted in sockets, cither loosely or confluent with the bony 

 walls of the cavity; these I have termed the "thecodont"* Lacertians: the 

 most ancient of all Saurians belong to this group. 



Thecodontosaurus, Riley and Stutchbury. — In the dolomitic conglomerate at 

 Redland near Bristol, a formation considered to belong to the oldest or lowest 

 division of the new red sandstone scries, remains of reptiles have been dis- 

 coveied by Dr. Riley and Mr. Stutchbury f, which are allied in the form of 

 their teeth to the typical Varanian Monitors, but differ in having the teeth 

 imbedded in distinct sockets ; to this condition, however, the Varani, among 

 the squamate Saurians, make an approacii in the shallow cavities containing 

 the base of the teeth along the bottom of the alveolar groove. 



In the ancient extinct genus in question the sockets are deeper, and the 

 inner alveolar wall is nearly as high as the outer one; the teeth are arranged 

 in a close-set series, slightly decreasing in size towards the posterior part of 

 the jaw ; each ramus of the lower jaw is supposed to have contained twenty- 

 one teeth. These are conical, rather slender, compressed and acutely pointed, 

 with an anterior and posterior finely-serrated edge, the serratures being di- 

 rected towards the apex of the tooth, as in the genus Bhopalodon of G. 

 I'ischer; 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. The pulp-cavity remains open in the base of the 

 crown. In their microscopic structure, the teeth of the Thecodontosaurus 

 closely correspond with that of tlie teeth of the Varamis, Monitor, and Me- 

 galosaurus. The body of the tooth consists of compact dentine, in which the 

 calcigerous tubes diverge from an open pulp-cavity at nearly right angles to 



* Odontography, part ii. p. 266. f Geological Transactions, 1836, p. 349. 



