TEETH 245 



Between the ordinary teeth of Snakes and the poison-fangs with closed 

 canals, there are numerous intermediate forms in which certain of the teeth 

 are simply grooved along their anterior side. A similar condition is also 

 seen in the teeth of the lower jaw of a poisonous Mexican Lizard (Heloderma). 

 (Comp. p. 252.) 



A peculiar tooth is present in the embryos of Lizards and some Snakes. 

 It projects considerably beyond its neighbours, and lies in the median line of 

 the lower jaAv, extending vertically towards the snout and serving the young 

 as a means of breaking through the parchment-like egg-shell. This must 

 not be confounded with the horny "neb" in Crocodiles, Chelonians, Birds, 

 and Monotremes amongst Mammals, which is of a purely epithelial nature. 



Chelonians, like existing Birds, are provided with horny sheaths 

 to the jaws instead of teeth. The presence of teeth in the embryo 

 of Trionyx, as well as of a rudimentary dental lamina in embryos of 

 Chelone and Sterna, for example, proves, however, that this is only 

 a secondary condition. 



In the cretaceous Birds of N. America (Odontornithes) teeth were 

 present, and were either situated in a definite alveoli (Ichthyornis), 

 or simply in grooves (Hesperornis). The premaxillse were tooth- 

 less, and seem to have possessed a horny beak. The single-pointed, 

 smooth teeth of Archaeopteryx were probably situated in alveoli. 

 It is possible that some of the Eocene Birds (e.g.> Argillornis, 

 Gastornis) possessed teeth. 



Mammals. The heterodonb dentition characteristic of the 

 Mammalia as a Class must have arisen by a modification of a 

 simple homodont condition, in which the teeth were all conical 

 and of similar size and shape. Side by side with this modifi- 

 cation, a shortening of the jaws has usually taken place, and 

 the teeth serve not only to seize and bite the food, but also to 

 masticate it and to test its qualities. The frequent presence of 

 rudimentary, functionless teeth, renders it probable that in the 

 course of phylogenetic development the teeth have undergone a 

 decrease in number. 1 An increase in number, such as is met with 

 in toothed Whales, is due to the separation, during ontogeny, of 

 the component cusps of complex teeth, and is therefore not a 

 primitive, but a highly specialised condition. 



As already mentioned, the succession is nearly always reduced 

 to two functional sets, the so-called milk or deciduous teeth and the 

 successional or permanent teeth, and in some cases (see p. 249) even 

 one of these may be rudimentary. Traces, however, of an earlier 

 set occur in certain Mammals : this may be spoken of as a " pre- 

 milk dentition." Occasionally also (e.g., in Man) one or more 

 teeth appear which replace the corresponding " permanent " teeth 

 and thus indications of four and possibly even of five sets can 

 in all be recognised. 



In each of the two functional sets, incisors, canines, and cheek- 



1 The last molar of Man, or so-called " wisdom-tooth," seems to be gradually 

 disappearing ; it appears last and is lost first, and often does not reach the 

 grinding surface. In many cases also the outer upper incisors are wanting. 



