892 



TEETH. 



All the teeth are attached, after the Pleuro- 

 dont type, by their base and outer margin to 

 shallow depressions on the outer side of the 

 external alveolar parapet. 



The germs of the successional teeth, c. 



Jig. 570., are developed at the inner side of the 



base of their predecessors, a, which they 



excavate, undermine, and displace in the 



usual manner. 



Iguanas. Certain genera of this family of 

 lizards, e. g. Istiiirus, Lophyrus, Calotes, and 

 Otocryptis, have the teeth soldered, like those 

 of Mosftsaurns, to the summit of the alveolar 

 ridge, and thence are called " Acrodonts : " in 

 all these lizards the maxillary and mandibular 

 teeth may be divided into anterior laniary, 

 and posterior molary teeth. In most of the 

 Iguaiiians the teeth are lodged in a common 

 shallow oblique alveolar groove, and are 

 .soldered to excavations on the inner surface 

 of the outer wall of the groove : these are 

 called Pleurodonts. Most of them possess 

 pterygoid as well as maxillary teeth ; but 

 the following genera, Jffyperanodon, Tropido- 

 lepis, Phrynosoma, and Callisaurus, are excep- 

 tions. 



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 mar- 

 gin of the crown. 



The Amblyrhynchus, a genus which is some- 

 what remarkable for the marine habits of at 

 least one of the species {Amblyrhynchus atcr), 

 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 tubernilala), is characterised 

 by the crenate or dentated margin of the 

 crown of the maxillary and premandibular 

 teeth, a few of the anterior small ones ex- 

 cepted. The pterygoid teeth are arranged 

 in two or three irregular rows, resembling 

 somewhat the " dents en cardes " of fishes. 

 In the full-grown Iguana tuberculata there 

 are from forty-seven to forty-nine teeth in 

 both upper and lower jaws. The number 

 is less in young subjects. The double row 

 of pterygoid teeth are in close order on each 

 side. 



In the horned Iguana (Metopoceros cor- 

 mdus), there are about fifty-six teeth in the 

 upper and lower jaws, of which the four 

 first are conical and slightly recurved. The 

 twelve succeeding teeth are somewhat larger 

 in size, with more compressed and expanded 

 crowns ; the rest are triangular, compressed, 

 with dentated margins. The inner surface of 

 the crown of the tooth is simply convex and 



* This species, and probably all Ilie known Am- 

 Uyrln/nchi, or blunt-nosed Iguana?, inhabit the 

 islands in the Galopagos group ; their habits have 

 been well elucidated by Mr. Danvin (Voyage of the 

 Beagle, vol. iii. p. dGG.). In specimens which he 

 dissected, he found the stomach loaded with minced 

 sea-weed. 



smooth ; the outer surface traversed by a me- 

 dian, longitudinal, broad, obtuse ridge. There 

 is a single row of small teeth implanted in 

 each pterygoid bone. No Iguanian lizard has 

 teeth on the palatine bones. 



The teeth of the Iguanodon, though re- 

 sembling 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, 

 still more essentially, in a modification of the 

 internal structure, by which the Iguanodon 

 equally 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 acu- 

 minated, compressed, its sloping sides ser- 

 rated, and its external surface traversed by 

 a median longitudinal 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. 

 In most of the teeth that have hitherto been 



Fie. 571. 



Unworn tooth of Iguanodon. 



found, three longitudinal ridges (fg. 571.) 

 traverse the outer surface of the crown, one 

 on each side of the median primitive ridye ; 

 these are separated from each other, and from 

 the serrated margins of the crown, by four 

 wide and smooth longitudinal grooves. The 

 relative width of these grooves varies in 

 different teeth ; sometimes a fourth small lon- 

 gitudinal ridge is developed on the outer 

 side of the crown. The marginal serrations, 

 which, at first sight, appear to be simple 

 notches, as in the Iguana, present, under 

 a low magnifying power, the form of trans- 

 verse ridges, themselves notched, so as to 

 resemble the mammilated margins of the un- 

 worn plates of the elephant's grinder: slight 

 grooves lead from the interspaces of these 

 notches upon the sides of the marginal ridges. 

 These ridges, or dentations, do not extend 

 beyond the expanded part of the crown : 

 the longitudinal ridges are continued further 

 down, especially the median ones, which do 

 not subside till the fang of the tooth begins 

 to assume its subcylindrical form. The tooth 

 at first increases both in breadth and thick- 

 ness ; it then diminishes in breadth, but 

 its thickness goes on increasing ; in the 

 larger and fully formed teeth, the fang de- 



