262 GEOSAURUS. 



size, at PI. 72, fig. 1 & 2. The teeth are about one half the size of 

 those of the Mosasaurus Hofmanni ; they differ in having their outer 

 side as convex as the inner side, the transverse section of the crown 

 being eUiptic with the ends of the elHpse pointed as in fig. 1 , the 

 points corresponding with two opposite longitudinal ridges, which 

 separate the outer from the inner side of the tooth : the tooth is very 

 slightly incurved ; the line of its anterior margin is convex, that 

 of the posterior one is nearly straight. The teeth are well 

 adapted, by their form, for piercing and cutting ; their sum- 

 mits are sharp- pointed, and their margins trenchant : the an- 

 terior edge is most produced. The crown of the tooth is compressed 

 but gradually expands, so that its base is circular, as represented at 

 fig. 1, h. This is supported, as in the Mosasaurus, upon a round 

 hillock of bone resting upon the broad alveolar surface of the jaw. 

 The close arrangement of the supporting bases of the teeth in this 

 acrodont reptile gives a crenate outline to the margin of the 

 jaw. 



The teeth, in the state in which they have come under my obser- 

 vation, exhibit very strikingly the lamellar decomposition of the den- 

 tine, upon which one of the arguments for the excretion- theory 

 has beenfounded(l). Fig. 2 shows the concentric arrangement of these 

 lamellse at the base of the crown of a fractured tooth, the centre of 

 which contains a wide pnlp-cavity, occupied by the chalk matrix : 

 this structure is also exhibited at fig. 1,6; it probably depends upon 

 the immature state of the tooth's formation. 



The crown of the tooth is defended, as in the Mosasaur, by a 

 coat of enamel. 



108. Geosaurus. — The teeth of an extinct reptile, whose large 

 eyes, defended by broad sclerotic plates, indicate the sea to have 

 been its abode, but which has received the name of Geosaurus from 

 Cuvier, resemble those of the large Varanian lizards in their com- 



(1) According to the Report in the Literary Gazette, 1839, Sept. 21, p. 598, this argument, 

 founded upon the appearances presented by the mammoth's tusks when in a state of lamellar 

 decomposition, was brought forward by the author of the paper on the structure and develop- 

 ment of teeth communicated to the British Association at Birmingham, in refutation of Dr. 

 Schwann's hypothesis of the formation of ivory by ossification of the pulp. The fallacy of the 

 argument was shown in my memoir in the ' Comptes Rendus' of the December following. 



