258 A MANUAL OF DENTAL ANATOMY. 



jaws the teeth might be grouped as incisors and canines, 

 so far as size and probable function go in such a classification. 



In structure the teeth of crocodiles consist of hard, fine 

 tubed dentine, with an investing cap of enamel, and in 

 addition a coating of cementum on their implanted por- 

 tions. As already mentioned, they are implanted in tubu- 

 lar sockets ; new successional teeth are being continually 

 developed at the inner side of their bases, and as these at- 

 tain to a certain size, absorption attacks the base of the 

 older tooth, and its successor moves into the space so gained? 

 so that it comes to be situated vertically beneath the older 

 tooth. In its further growth it causes yet more absorption 

 of the older tooth, which it ultimately pushes out in front 

 of it, sometimes carrying the remains of the old tooth like 

 a cap upon its own apex when it first emerges. Each new 

 tooth vertically succeeds its predecessor ; hence no additional 

 teeth are added, but the young newly hatched crocodile has 

 as many teeth as a full grown one. 



In the extinct Ichthyosaurus the teeth, while forming an 

 armature not unlike that of some of the crocodiles, were 

 not implanted in distinct sockets, but were lodged in a 

 continuous shallow groove, with but slight indications of 

 transverse divisions. 



The huge Dinosauria, some of which must have been 

 thirty feet in length, had teeth implanted in imperfect 

 sockets, the outer alveolar wall being considerably higher than 

 the inner, and the transverse septa not very complete. The 

 roots of the teeth were more or less perfectly cylindrical, 

 and the enamelled crowns compressed and expanded, with 

 trenchant edges. The tooth of the Iguanodon will serve as a 

 fair example of a Dinosaurian tooth : the crown is greatly ex- 

 panded, and presents anterior and posterior sharp notched 

 margins ; the enamel is laid over the outer surface of upper 

 teeth, and the inner of lower teeth. The enamelled surface 

 is ridged, so that as it wears down a notched edge is main- 



