182 ATTACHMENT. 



form as yet discovered in the class of reptiles. In no reptile does 

 the base of the tooth ever branch into fangs. 



71. Attachment. — As a general rule, the teeth of reptiles are anchy- 

 losed to the bone which supports them. When they continue distinct, 

 they may be lodged either in a continuous groove, as in the Ichthyo- 

 saur, or in separate sockets, as in the Plesiosaur and Crocodilians. 

 The base of the tooth is anchylosed to the walls of a moderately deep 

 socket in the extinct Megalosaur and Thecodon. In the Labyrintho- 

 dons and Caecilise among the Batrachians ; in most Ophidians ; and 

 in the Geckos, Agamians, and Varanians, the base of the tooth is 

 imbedded in a shallow socket and confluent therewith(l). In the 

 Scincoidians, Safe-guards {Tejus), in most Iguanians, in the Cha- 

 meleons and most other Lacertian reptiles, the tooth is anchylosed 

 by an oblique surface extending from the base more or less upon the 

 outer side of the crown to an external alveolar plate of bone, as 

 shown in Plate 67 ; the inner alveolar plate not being developed. In the 

 frogs, the teeth are similarly but less firmly attached to an external 

 parapet of bone. The lizards which have their teeth thus attached 

 to the side of the jaw are termed * Pleurodonts.' In a few Iguanians, 

 as the Istiures, the teeth appear to be soldered to the margins of the 

 jaws, these have been termed ' Acrodonts.' In some extinct Lacer- 

 tians, as the Mosasaur and Leiodon, the tooth is fixed upon a raised 

 conical process of bone, as shown in Plate 68, fig. 1 and Plate 72, 

 fig. 2. 



These modifications of the mode of attachment of the teeth of rep- 

 tiles are closely adapted to the destined application of those instruments 

 and to the habits of the species ; we may likewise perceive that they offer 

 a close analogy to some of the transitory conditions of the human teeth. 

 There is a period, for example, (2) when the primitive dental pa- 

 pillae are not defended by either an outer or an inner alveolar 

 process, any more than their gigantic calcified analogues in the 

 extinct Mosasaur. There is another stage(3) in which the groove 



(1) PI. 63 A, fig. 8. a a, sockets of the anchylosed teeth in Varanus varius. 



(2) At the sixth month, see Mr. Goodsir, On the development of the Human Teeth.— Edin- 

 burgh Medical and Surgical Journal, No. 138. 



(3) At the seventh or eighth week. — Ibid. 



