TEETH OF REPTILES. 403 



Hyper anodon, Tropidolepis, Phrynosoma, and Callisaurus, are 

 exceptions. 



In the Pleurodont Iguanians, the teeth never present the true 

 laniary form ; and, if simply conical, as at the extremes of the 

 maxillary series, the cone is more or less obtuse ; but, in general, 

 it is expanded, more or less trilobate, or dentated along the margin 

 of the crown. The Amblyrhynchus, a genus remarkable for the 

 marine habits of at least one of the species (Amblyrhynchus ate?'), 

 whose diet is sea-weed, has the tricuspid structure well developed 

 in the posterior teeth. The typical genus of the present family 

 of Saurians (Iguana tuherculata) is characterised by the crenate 

 or dentated margin of the crown of the maxillary and premandi- 

 bular teeth, a few of the anterior small ones excepted. The ptery- 

 goid teeth are arranged in two or three irregular rows, resembling 

 somewhat the ' dents en cardes ' of Fishes. In the full-grown 

 Iguana tuherculata there are from forty-seven to forty-nine teeth 

 in both upper and lower jaws. The number is less in young 

 subjects. The two rows of pterygoid teeth are in close order 

 on each side. In the horned Iguana (Metopoceros cornutus) there 

 is a single row of small teeth implanted in each pterygoid bone, 

 fig. 98, D, 24. 



The teeth of the Iguanodon, though resembling those of the 

 Iguana, do not present an exact magnified image of them, but 

 differ in the greater relative thickness of the crown, its more 

 complicated external surface, and in a modification of the internal 

 structure, by which this huge herbivorous extinct lizard deviates 

 from every other known reptile. 



As in the Iguana, the base of the tooth is elongated, contracted, 

 and subcylindrical ; the crown expanded, and smoothly 

 convex on the inner side. When first formed, it is 

 acuminate, compressed, its sloping sides serrate, and 

 its external surface traversed by a median longitu- 

 dinal ridge, and coated by a layer of enamel; but, 

 beyond this point, the description of the tooth of the 

 Iguanodon indicates characters peculiar, to that genus. 

 Three longitudinal ridges, fig. 273, traverse the outer 

 surface of the crown, one on each side of the median 

 primitive ridge ; these are separated from each other, unwo^f tooth 

 and from the serrated margins of the crown, by four of I e^<" ion - 

 wide and smooth longitudinal grooves. In the upper jaw the 

 teeth are less curved, and are thicker transversely to the jaw : 

 the primary ridge is much more prominent. The marginal ser- 

 rations present, under a low magnifying power, the form of 



D D 2 



