18G DEVELOPMENT. 



is not unusual to find the capsules of contiguous tooth-germs becoming 

 adherent together as their ossification proceeds. After a brief mace- 

 ration the soft gum may be stripped from the shallow alveolar depres- 

 sion 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 developed. 



The situation in which the successional teeth are developed in 

 the Varanus is shown in PL 63 a, fig. 9 : their relative position to, 

 and the mode in which they affect, the adherent teeth, by pressing 

 upon the inner side of their bases, is shown in the upper and lower 

 jaws of the Cyclodus (Plate 66, fig. 7 b) : the phenomena of dental 

 development in these and other lizards closely correspond with those 

 described in the frog. In the Acrodont lizards, and those in which 

 the teeth are anchylosed to the summits of bony processes, the suc- 

 cessional teeth are in like manner developed at the inner side of 

 the supporting processes, gradually penetrate them as their growth 

 proceeds, and finally undermine and displace the tooth and become 

 in their turn anchylosed to new bony eminences of the alveolar 

 tract. The jaws of the gigantic Mosasaur exhibit on a large scale 

 different stages of this mode of shedding and replacement, which 

 is so general in the class of reptiles. 



In the Ichthyosaurus, in which, by the development of an internal 

 as well as an external alveolar plate, the teeth are lodged in a deep 

 continuous groove, the successional germs were also developed in this 

 extinct reptile at the inner side of their predecessors, and, from the 

 solidification of the implanted base of the fully formed tooth, occa- 

 sioned an extensive absorption of its inner side, before it finally 

 yielded to the lateral pressure. 



In the Crocodile, the tooth-germ is developed 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 (PI. 75, 

 tig. 4). In this alveolus, however, it never becomes inclosed hke the 



