890 



TEETH. 



upper jaw is trenchant, and seems to have 

 been sheathed with horn. 



The maxillary bone (21) is excavated by a 

 wide and deep alveolus, with a circular area 

 of half an inch, and lodges a long and strong, 

 slightly curved, and sharp-pointed, canine tooth 

 or tusk, which projects about two-thirds of its 

 length from the open extremity of the socket. 

 The direction of the tusks is forwards, down- 

 wards, and very slightly inwards ; the two 

 converging, as they descend along the outer 

 side of the compressed symphysis of the 

 lower jaw (c). The tusk is principally 

 composed of a body of compact unvascular 

 dentine. The base is excavated by a wide 

 conical pulp-cavity (j)) with the apex extend- 

 ing to about one-half of the implanted part 

 of the tusk, and a linear tract is continued 

 along the centre of the solid part of the 

 tusk. From this central line the dentinal tubes 

 radiate, with a gentle curve at the beginning, 

 convex towards the point of the tusk, and then 

 proceeding straight to the periphery of the 

 tooth, but inclining towards the apex. They 

 present parallel secondary curves, divide di- 

 chotomously twice or thrice near their begin- 

 ning, and send off numerous small lateral 

 branches, chiefly from the side next the apex. 

 At their primary curve the dentinal tubes are 

 __^__th of an inch in diameter, and their 

 intervals are ^-o^jth of an inch across. The 

 dentinal cells are most conspicuous near the 

 periphery of the tooth, and vary in diameter 

 from -g^th to T7 ^-oth of an inch. 



The enamel, at least at the middle of the 

 tusk, is thinner than in the teeth of the cro- 

 codile. It presents only a finely lamellated 

 texture, the layers being parallel with the 

 surface of the dentine on which it rests. 

 There is only a fine linear trace of cement on 

 the exterior of the sections of the implanted 

 base of the tusks ; and here it is too thin to 

 allow of the development of the radiated cells 

 in its substance. There is no trace of teeth 

 or their sockets in the lower jaw (25, 23); so 

 much of the alveolar border as is exposed pre- 

 sents a smooth and even edge, which seems to 

 have played like a scissor-blade upon the inner 

 side of the corresponding edentulous border 

 of the upper jaw ; and it is most probable, 

 from the analogies of similarly-shaped jaws of 

 existing Reptilia, that the fore part of both 

 the upper and under jaws were sheathed with 

 horn. 



Until the discovery of the Rhynchosaurus*, 

 this edentulous and horn-sheathed condition 

 of the jaws was supposed to be peculiar to 

 the Chelonian order among reptiles ; and it is 

 not one of the least interesting features of 

 the Dicynodonts of the African sandstones, 

 that they should repeat a Chelonian character, 

 hitherto peculiar, amongst Lacertians, to the 

 above-cited remarkable extinct edentulous 

 genus of the new red sandstone of Shrop- 

 shire: but our interest rises almost to as- 

 tonishment, when, in a Saurian skull, we 

 find, superadded to the horn-clad mandibles 



* Transactions of the Cambridge Philosophical 

 Society, vol. vii. part iii. 



of the Tortoise, a pair of tusks, borrowed as 

 it were from the mammalian class, or rather 

 foreshadowing a structure which, in the actual 

 creation, is peculiar to certain members of the 

 highest organised warm-blooded animals. 



In the other Reptilia, recent or extinct, 

 which most nearly approach the Mammalia in 

 the structure of their teeth, the difference 

 characteristic of the inferior and cold-blooded 

 class is manifested in the shape, and in the 

 system of shedding and succession, of the 

 teeth : the base of the implanted teeth seldom 

 becomes consolidated, never contracted to a 

 point, as in the fangs of the simple teeth of 

 Mammalia, and at all periods of growth one 

 or more germs of teeth are formed within or 

 near the base of the tooth in use, prepared to 

 succeed it, and progressing towards its dis- 

 placement. The dental armature of the jaws 

 is kept in serviceable order by uninterrupted 

 change and succession ; but the matrix of the 

 individual tooth is soon exhausted, and the 

 life of the tooth itself may be said to be com- 

 paratively short. 



The Dicynodonts not only manifest the 

 higher type of free implantation of the base 

 of the tooth in a deep and complete socket, 

 common to Crocodilians, Megalosaurs, and 

 Thecodonts, but make an additional and much 

 more important step towards the mammalian 

 type of dentition, by maintaining the service- 

 able state of the tusk by virtue of constant 

 renovation of the substance of one and the 

 same matrix, accordingly to the principle 

 manifested in the long-lived and ever-growing 

 tusks of the Walrus, and the scalpriform in- 

 cisors of the Rodentia. 



The genera of the typical family of the 

 squamate Lacertians are arranged in two 

 sub-families, the chief characteristics of which 

 are derived from the dental system. 



In the first group, the teeth are solid, or 

 without any permanent internal cavity, and 

 are very firmly anchylosed by their base to 

 the alveolar groove upon the inner side of 

 the jaw ; so that the extremity of the tooth 

 is slightly directed outwards. The species 

 which present this character are called Pleo- 

 donts. 



In the second group, the teeth are ex- 

 cavated, or retain the pulp-cavity, and are 

 less firmly fixed to the jaws, being applied 

 vertically, like piles or buttresses, against the 

 outer alveolar parapet, but not adhering by 

 their base. This group is called Coelodonts. 



The Monitor Lizard of S. America is an 

 example of the Pleodont group, in which the 

 premaxillary teeth are ten in number. The 

 maxillary teeth vary from ten to fifteen on 

 each side, and increase in size as they are 

 placed farther back : the hindmost teeth are 

 tricuspid in young individuals, and present 

 the form of simple tubercles in the old Mo- 

 nitors. The mandibular teeth, fifteen to 

 eighteen in number in each ramus, correspond 

 in size and form with those above. In the 

 Coelodont group, the "Swift lizards" (Tachy- 

 dromus) have the pterygoid bones armed with 

 minute teeth. The teeth on both upper and 



