884* 



TEETH. 



imbedded in a shallow socket, and is confluent 

 therewith. 



In the Scincoidians, the Safeguards (Tejus\ 

 in most Iguanians, in the Chameleons 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*, the inner alveolar plate not being 

 developed. In the frogs the teeth are simi- 

 larly 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 Igua- 

 nians, as the Istiures, the teeth appear to be 

 soldered to the margins of the jaws, these 

 have been termed " Acrodonts." In some 

 large extinct Lacertians, e. g. the Mosasaur 

 and Leiodon, the tooth is fixed upon a raised 

 conical process of bone, as shown in my 

 " Odontography," Plate 68. fig. 1., and Plate 

 72. fig. 2. 



These modifications of the attachment of 

 the teeth of reptiles are closely adapted to the 

 destined application of those instruments, and 

 relate to the habits and food of the species ; 

 we may likewise perceive that they offer a 

 close analogy to some of the transitorv con- 

 ditions of the human teeth. There is a 

 period, for example f, when the primitive 

 dental papilla are not defended by either an 

 outer or an inner alveolar process, any more 

 than their calcified homologues which are 

 confluent with the margin of the jaw in the 

 Rhynchocephalus.\ There is another stage , 

 in which the groove containing the den- 

 tal germs is defended by a single external 

 cartilaginous alveolar ridge ; this condition is 

 permanently typified in the Cydodus (fig. 57tX) 

 and most existing lizards. Next there is 

 developed in the human embryo an internal 

 alveolar plate, and the sacs and pulps of the 

 teeth sink into a deep but continuous groove, 

 in which traces of transverse partitions soon 

 make their appearance ; in the ancient Ichthy- 

 osmir the relation of the jaws to the teeth 

 never advanced beyond this stage. 



Finally, the dental groove is divided by 

 complete partitions ||, and a separate socket 

 is formed for each tooth ; and this stage of 

 developement is attained in the highest or- 

 gan^sed reptiles, e.g. the crocodiles (fig. 573.). 

 ^ Substance. This may be four-fold, and a 

 single tooth may be composed of dentine, 

 cement, enamel, and bone; but the dentine 

 and cement are present in the teeth of all 

 reptiles. 



In the Batrachians and Ophidians a thin 

 layer of cement invests the central body 

 of dentine, and, as usual, follows any in- 

 flections or sinuosities that may characterise 



* Odontography, pi. 67. 



f At the sixth week of gestation : see Prof. Good- 

 Sj'- 1 V 1 ^ Devel P men t of the Human Teeth," 

 Edinburgh Medical and Surgical Journal, No 138 



J See Geological Transactions, 2d series, vol. vii. 

 pt. 2, pi. ,fi gs . 5 & 6, p. 83. 



At the seventh or eighth week : Ibid 



|| At the sixth month : Ibid. 



the dentine. Besides the outer coat of cement, 

 which is thickest at the base of the teeth, a 

 generally thin coat of enamel defends the 

 crown of the tooth in most Saurians, and the 

 last remains of the pulp are not unfrequently 

 converted into a coarse bone, both in the 

 teeth which are anchylosed to the jaw, and 

 in some teeth, as those of the Ichthyosaur, 

 which remain free. The only modification of 

 the dentine, which could at all entitle it to be 

 regarded in the light of a new or distinct 

 substance, is that which is peculiar in the 

 present class to the teeth of the Iguanodon, 

 and which will be described in the following 

 section. 



Structure. The varieties of dental structure 

 are few in the reptiles as compared with 

 either fishes or mammals, and its most com- 

 plicated condition arises from interblending of 

 the dentinal and other substances rather than 

 from modifications of the tissues themselves. 

 In the teeth of most reptiles the intimate 

 structure of the dentine corresponds with 

 that which has been described as the type of 

 the tissue, e.g. the hard or unvascular dentine, 

 and which is the prevailing modification in 

 Mammalia, viz., the radiation of a system of 

 minute plasmatic tubes from a single pulp- 

 cavity, at right angles to the external surface 

 of the tooth. The most essential modification 

 of this structure is the intermingling of cylin- 

 drical processes of the pulp-cavity in the form 

 .of medullary canals, with the finer tubular 

 structure.* Another modification is that in 

 which the dentine maintains its normal struc- 

 ture, but is folded inwardly upon itself, so as 

 to produce a deep longitudinal indentation on 

 one side of the tooth ; it is the expansion of 

 the bottom of such a longitudinal deep fold 

 that forms the central canal of the venom- 

 fang of the serpent ; but a glance at fig. 568. 

 will show that, notwithstanding the singularly 

 modified disposition of the dentine (b), its 

 structure remains unaltered ; and although 

 the pulp-cavity (p] is reduced to the form of a 

 crescentic fissure, thedentinal tubes continue to 

 radiate from it according to the usual law. By 

 a similar inflection of many vertical longi- 

 tudinal folds of the external cement and ex- 

 ternal surface of the tooth at regular intervals 

 around the entire circumference of the tooth, 

 and by a corresponding extension of radiated 

 processes of the pulp-cavity and dentine into 

 the interspaces of such inflected and con- 

 verging folds, a modification of dental struc- 

 ture is established in certain extinct reptiles, 

 which, by the various sinuosities of the inter- 

 blended folds of cement and processes of 

 dentine, with the partial dilatations of the 

 radiated pulp-cavity, produces the compli- 

 cated structure which is described at p. 868. 

 and figured in cut 552. But this compli- 

 cation is nevertheless referable to a modi- 

 fication of form or arrangement of the dental 

 tissues, rather than of the structure of the 

 tissues themselves: the calcigerous tubes in 

 each sinuous lobe of dentine, in the most 



* Odontography, pi. 71, Iguanodon. 



