TEETH. 



889 



the turgescent duct with which it is in con- 

 tact, the tubes which proceed to the surface 

 d, while maintaining their normal relation of 

 the right angle to it, are extremely short ; and 

 the layer of dentine separating the poison- 

 tube from the pulp-cavity is proportionally 

 thin. The calcigerous tubes that radiate from 

 the opposite side of the pulp-cavity to the 

 exposed surface b of the tooth are dispropor- 

 tionally long. 



The teeth of Ophidians are developed and 

 completed in that part which forms the ori- 

 ginal seat of the tooth-germs in all animals ; 

 viz. the mucous membrane or gum covering, 

 the alveolar border of the dentigerous bones. 

 This germ presents the same lax tissue, and 

 is as abundantly developed, as in the Pike, 

 Lophius, and many other fishes ; in which it 

 likewise serves as the nidus and locality for 

 the complete development of the teeth. The 

 primitive dental papillae in the common harm- 

 less snake very soon sinks into the substance 

 of the gum, and becomes inclosed by a cap- 

 sule. As soon as the deposition of the cal- 

 careous salts commences in the apex of the 

 papilla the capsule covering that part becomes 

 ossified and adherent to the dentine, and the 

 tooth begins to pierce and emerge from the 

 gum before its mould, the pulp, is half com- 

 pleted. Fresh layers of cells are successively 

 added to the base of the pulp, and converted, 

 by their confluence and calcification, into the 

 tubular dentine, until the full size of the 

 tooth is attained, when its situation in the 

 gum is gradually changed, and its base becomes 



surface of the pulp ; and the base of the 

 groove of the loose, growing, poison-fang is 

 brought into the same relation with the duct 

 of the poison gland as the displaced fang, 

 which has been severed from the duct. 



Saurians. The existing species of lizards 

 differ from those of the crocodile in the anchy- 

 losed condition of the teeth, which present few 

 modifications of importance ; those that yield 

 most fruit to physiology, and which have most 

 expanded our ideas of the extent of the re- 

 sources of Nature and the exceptional devi- 

 ations from what was deemed the rule of 

 structure in the Saurian dentition, have been 

 discovered by the study of the fossil teeth of 

 extinct forms of the order. Amongst these the 

 most extraordinary in respect of their dental 

 system have been recently discovered in aform- 

 ation in South Africa, which seems nearly as 

 ancient as our own coal-seams. I have called 

 them " Dicynodonts,"* from their dentition 

 being reduced to one long and large canine 

 tooth on each side of the upper jaw. As 

 these teeth give, at first sight, a character to 

 the jaws like that which the long poison-fangs 

 give, when erected, to the jaws of the rattle- 

 snake, I shall briefly notice their characters 

 before entering upon the description of the 

 more normal Saurian dentition. 



Fig. 569. gives a reduced side view of the 

 skull of the species of Dicynodon called D. la- 

 certiceps. The cranial cavity (8, 8) is extremely 

 contracted, as in all the cold-blooded quad- 

 rupeds : it is bounded on each side by wide 

 and deep temporal fossae (t) indicating power- 



Fig. 569. 



Skull of Dicynodon lacerticeps, one-third natural size. 



anchylosed to the shallow cavity of the alve- 

 olar surface of the bone. 



In the posterior part of the large mucous 

 sheath of the poison-fang, the successors of 

 this tooth are always to be found in different 

 stages of development ; the pulp is at first 

 a simple papilla, and when it has sunk into 

 the gum the succeeding portion presents a 

 depression along its inferior surface, as it lies 

 horizontally, with the apex directed back- 

 wards; the capsule adheres to this inflected 



ful muscles for the action of the lower jaw. 

 The orbits (o) are large and round ; the nos- 

 trils (H) are divided by the junction of the 

 nasal bones (15) with the premaxillaries (22), 

 as in lizards ; there is not a single median 

 external nostril, as in Chelonian and Croco- 

 dilian reptiles. The alveolar border of the 

 lower jaw and of the premaxillary part of the 



* From Si;, two, and xwiZous, the name given by 

 Hippocrates to the canine teeth, and signifying the 

 same idea as their common English denomination. 



