288 



REPT1LIA. 



coats of the oesophagus : their extremities, 

 which are thus introduced into the alimentary 

 canal, are coated with a layer of hard dentine, 

 and form substitutes for the teeth, which, if 

 not always entirely absent, are merely ru- 

 dimental in the ordinary situations in the 

 mouth. 



In the tortoises and turtles the jaws are 

 covered, as is well known, by a sheath of 

 horn, which in some species is of considerable 

 thickness, and very dense ; its working sur- 

 face is trenchant in the carnivorous species, 

 but variously sculptured and adapted for 

 both cutting and bruising in the vegetable 

 feeders. 



The development of the continuous horny 

 maxillary sheath commences, as in the parrot 

 tribe, from a series of distinct papillae, which 

 sink into alveolar cavities, regularly arranged 

 (in Trionyx) along the margin of the upper 

 and lower jaw-bones. These alveoli are indi- 

 cated by the persistence of vascular canals 

 long after the originally separate tooth-like 

 cones have become confluent and the horny 

 sheath completed. 



The teeth of the dentigerous Saurian and 

 Ophidian reptiles are, for the most part, 

 simple, and adapted for seizing and holding, 

 but not for dividing or masticating their food. 



In no reptile are the teeth reduced to so 

 small a number as in certain mammals and 

 fishes, nor are they ever so numerous as in 

 many of the latter class. Some species of 

 Monitor ( Varanus) with sixteen teeth in the 

 upper and fourteen in the lower jaw, afford 

 examples of the smallest number in the 

 present class. It is rarely that the number 

 of teeth is fixed and determinate in any 

 reptile so as to be characteristic of the 

 species. 



The teeth may be present on the jaws only, 

 as in the Crocodiles and many Lizards; or 

 upon the jaws, and roof of the mouth, and 

 here either upon the pterygoid bones, as in 

 the Iguana, or upon both palatine and ptery- 

 goid bones, as in most serpents. 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 extinct Ichthyosaur, or in separate sockets, 

 as in the Crocodilians. The base of the 

 tooth is anchylosed to the walls of a mode- 

 rately deep socket, in the extinct Megalosaur 

 and Thecodon. In most Ophidians, and in the 

 Geckos, Aganuans, and Varanians, the base 

 of the tooth is imbedded in a shallow socket, 

 and confluent therewith. In the Scincoidians, 

 Safe-guards (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. 



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 La- 

 certians, as the Mososaur and Leiodon, the 

 tooth is fixed upon a raised conical process 

 of bone. 



The completion of a tooth is soon followed 

 by preparation for its removal and succession. 

 The facility of developing new tooth germs 

 seems to be unlimited in the present class, 

 and the phenomena of dental decadence and 

 replacement are manifested at every period 

 of life. The number of teeth is generally 

 the same in each successive series, and 

 the difference of size presented by the 

 teeth of different and distinct series is consi- 

 derable. 



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. 



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 occupies the position of its 

 predecessor. 



In the crocodile the tooth -germ is deve- 

 loped from the vascular membrane covering 

 the base of the internal wall of the socket. 

 It is soon invested by a capsule, and by its 

 pressure causes the formation of a shallow 

 recess, or secondary alveolus, in the contiguous 

 bone. In this alveolus, however, it never 

 becomes inclosed like the successional teeth 

 in most mammalia ; for, exerting equal pres- 

 sure against the fang of the contiguous tooth, 

 which, from being incompletely formed, has 

 a wide pulp cavity with very thin walls, the 

 nascent tooth soon penetrates that cavity, 

 and quits the recess in the alveolar plate, in 

 which it was originally situated. Thus the 

 stage of development corresponding with the 

 eruption of the tooth in the mammalia is im- 

 mediately followed by the inclusion of the 

 new tooth in the pulp cavity of its prede- 

 cessor. The rapid succession of tooth germs, 

 which stamps the impress of decay upon their 

 predecessors often before the growth of these 

 is completed, though common to many rep- 

 tiles, is most strikingly manifested in the 

 crocodiles, in which three and sometimes 

 four generations of teeth, sheathed one 

 within the other, are contained in the same 

 socket. 



The order Ophidia, as it is characterised in 

 the system of Cuvier, requires to be divided 

 into two sections, according to the nature of 

 the food, and the consequent modification of 

 the jaws and teeth. Certain species, which 

 subsist on worms, insects, and other small 

 invertebrate animals, have the tympanic pedi- 

 cle of the lower jaw immediately and im- 

 moveably articulated to the walls of the 

 cranium. The lateral branches of the lower 



