406 ANATOMY OP VERTEBRATES. 



ventricose base, or implanted fang. They are confined to the 

 premaxillary, maxillary, and premandibular bones, in which they 

 are arranged in close series, and of nearly equal size. They 

 consist of a body of unvascular dentine, invested at the base by a 

 thick layer of cement, and at the crown by a layer of enamel, 

 which is itself covered by a very thin coat of cement ; the pulp- 

 cavity is more or less occupied in fully formed teeth by a coarse 

 bone. The external surface of the tooth is marked by the longi- 

 tudinal impressions and ridges, but the teeth vary both as to 

 outward sculpturing and general form in the different species. 1 

 The chief peculiarity of the dental system of the Ichthyosaur 

 is the mode of the implantation of the teeth : instead of being 

 anchylosed to the bottom and side of a continuous shallow groove, 

 as in most Lacertians, or implanted in distinct sockets, as in the 

 Thecodon, Megalosaur, or Pterodactyle, they are lodged loosely 

 in a long and deep continuous furrow, and retained by slight 

 .ridges between the teeth, along the sides and bottom of the 

 furrow, and by the gum and organised membranes continued into 

 the groove and upon the base of the teeth. The germs of the 

 new teeth are developed at the inner side of the base of the 

 old ones. 



The best and most readily recognisable characters by which 

 the existing Crocodilians are grouped in appropriate genera are 

 derived from modifications of the dental system. 



In the Caimans (genus Alligator} the teeth vary in number 



„ 18—18 4 22—22 . i^,, 



trorn — — — to — — — : the fourth tooth of the lower law, or 

 18 — 18 22 — 22 J 



canine, is received into a cavity of the palatal surface of the upper 



jaw, where it is concealed when the mouth is shut. In old 



individuals the upper jaw is perforated by these large inferior 



canines, and the fossa? are converted into foramina. 



In the Crocodiles (genus Crocodilus) the first tooth in the lower 

 jaw perforates the palatal process of the premaxillary bone when 

 the mouth is closed ; the fourth tooth in the lower jaw is received 

 into a notch excavated in the side of the alveolar border of the 

 upper jaw, and is visible externally when the mouth is closed. 



In the two preceding genera the alveolar borders of the jaw 

 have an uneven or wavy contour, and the teeth are of an unequal 

 size. 



In the Gavials (genus Gavialis) the teeth are nearly equal in 

 size and similar in form in both jaws, and the first as well as the 



1 v. pi. 73. 



