TEETH. 



885 



complex tooth of the Labyrinthodon, exhibit 

 the same general disposition and course as in 

 the fang of the serpent and in the still more 

 simple tooth of the Saurian. 



Development. The teeth of reptiles are 

 never completed, as in certain fishes, at the 

 first or papillary stage ; but the pulp sinks 

 into a follicle, and becomes inclosed by a cap- 

 sule; and in certain reptiles this becomes 

 more or less surrounded by bone ; but the 

 process of development never offers the erup- 

 tive stage, in the sense in which this is 

 usually understood, as signifying the extrica- 

 tion of the young tooth from a closed alveolus. 

 The completion of a tooth, with the extinct 

 exception of the Dicynodont Reptiles, is soon 

 followed by preparation for its removal and 

 succession : the faculty of developing new 

 tooth-germs seems to be unlimited in the pre- 

 sent class, and the phenomena of dental deca- 

 dence and replacement are manifested at every 

 period of life; the number of teeth is gene- 

 rally the same in each successive series, and 

 the difference of size presented by the teeth 

 of different and distant series is considerable. 

 The new germ is always developed, in the 

 first instance, at the side of the base of the 

 old tooth, never in the cavity of the base ; the 

 crocodiles form no exception to this rule. 

 The poison-fangs of serpents succeed each 

 other from behind forwards ; in almost every 

 other instance the germ of the successional 

 tooth is developed at the inner side of the 

 base of its predecessor. In the frog the 

 dental germ makes its appearance in the 

 form of a papilla developed from the bottom 

 and towards the outer side of a small fissure 

 in the mucous membrane or gum that fills up 

 the shallow groove at the inner side of the 

 alveolar parapet and its adherent teeth : the 

 papilla is soon enveloped by a capsular pro- 

 cess of the surrounding membrane : there is 

 a small enamel pulp developed from the cap- 

 sule opposite the apex of the tooth ; the de- 

 position of the earthy salts in this mould is 

 accompanied by ossification of the capsule, 

 which afterwards proceeds part passu with 

 the calcification of the dentinal papilla or 

 pulp; so that, with the exception of its base, 

 the surface of the uncalcified part of the pulp 

 alone remains normally unadherent to the cap- 

 sule. 



As the tooth acquires hardness and size, it 

 presses against the base of the contiguous 

 attached tooth, causes a progressive absorp- 

 tion of that part, and finally undermines, dis- 

 places, and replaces its predecessor. The 

 number of nascent matrices of the successional 

 teeth is so great in the frog, and they are 

 crowded so close together, that it is not 

 unusual to find the capsules of contiguous 

 tooth-germs becoming adherent together, as 

 their ossification proceeds. After a brief ma- 

 ceration, the soft gum may be stripped from 

 the shallow alveolar depression, and the 

 younger tooth-germs in different stages of 

 growth are brought away with it. 



The mode of development of the teeth of 

 serpents does not differ essentially from that 



of the teeth of the Batrachian above described 

 except in the relation of the papillae of the 

 successional poison-fangs to the branch of the 

 poison-duct that traverses the cavity of the 

 loose mucous gum in which they are deve- 

 loped. 



Batrachian modifications. Some of the pe- 

 culiarities of the dentition of the Batrachians 

 have already been noticed, as in the compa- 

 rison of the Siren with the Lepidosiren* in 

 which the true amphibian was shown to have 

 numerous teeth on the palate and lower jaw.* 

 The piscine character of rasp-like teeth 

 aggregated in numerous series, is manifested 

 also in the Axolotl f, upon the palatal region 

 of the mouth, and upon the splenial or oper- 

 cular element of the lower jaw ; but the 

 superior maxillary bones are here developed, 

 and also support teeth. The premandibular 

 and the premaxillary bones, instead of pre- 

 serving the larval condition of the horny 

 sheath, have their alveolar border armed with 

 a single row of small, equal, fine and sharp- 

 pointed denticles, which are continued above, 

 along the maxillaries; thus establishing the 

 commencement of the ordinary Batrachian 

 condition of the marginal teeth of the buccal 

 cavity. The dentigerous bones of the palate 

 consist of two plates on each side, as in the 

 Siren ; the anterior pair, or vomerine bones, 

 converge and meet at their anterior extre- 

 mities ; the minute denticles which they sup- 

 port are arranged quincuncially ; the posterior 

 pair of bones are continued backwards ac- 

 cording to the usual disposition of the ptery- 

 goids, to abut against the tympanic bones ; 

 the denticles are confined to the anterior part 

 of their oral surface, and resemble in their 

 arrangement and anchylosed attachment those 

 of the vomerine series, of which they form the 

 posterior termination. 



The frogs (Rana^) J have no teeth on the 

 lower jaw ; but in some species the alveolar 

 edge of this bone is finely notched or dentated, 

 as in the horned frogs {Ceratophrys). The in- 

 termaxillary and maxillary bones support a 

 long, close-set, single series of small, conical, 

 hollow teeth, of which the apices only project 

 beyond the external alveolar ridge to which 

 they are attached. A short transverse row 

 of similar but smaller teeth extends along the 

 posterior border of each vomer, except in the 

 slender-armed frogs (Leptobrachium), and in 

 some of the tree frogs (e. g . Euclinemis), in 

 which the roof of the mouth is edentulous. 



Amongst the most extraordinary examples 

 of extinct reptiles are those which are charac- 

 terised by the labyrinthic modification of the 

 dental structure above described, and which 

 with some affinities to Saurians, combine 

 characters which are essentially those of the 

 order Batrachia. I have ascertained by fossil 

 portions of the upper jaw of the Labyrinthodon 

 Icptognathus that the maxillary or facial divi- 

 sion of the skull was broad, much depressed, 

 and flattened, resembling the skull of the 



* Odontograpliy, pi. G'2,fys. 5 & G 

 t Ib. pi. 02,/</. 4. 

 J Ib. pi. M,fig. 10. 



3 L 3 



