106 The Rev W. D. Conybeare's Additional Notices 



the dentition of the crocodile differs from that of the other members of the 

 ^reat Saurian family. 



In the crocodile^ the teeth are lodged loosely in distinct alveoli: they always 

 remain hollow: the new tooth first appears as a germ on the inner face of the 

 root of the old toothy where, preventing- the growth, it occasions a fissure; 

 through which, as it continues to elongate itself, it penetrates into the hollow 

 of the old tooth : and, lastly, by its increase it splits the old tooth, the fragments 

 of which it causes to fall off. There being in the crocodile a continual suc- 

 cession of fresh teeth, and not a single change as in man (and as in most, I be- 

 lieve, of the mammalia), the teeth are never filled up by the ossification of their 

 pulpy interior, but always continue hollow: — and the process above described 

 may be traced as going on in the jaws of crocodiles of all ages. 



In the other laccrtas, on the other hand, the teeth are not lodged in alveoli, 

 nor even in a continuous furrow ; but the jaw bone presents only (if the ex- 

 pression may be allowed) a sort of parapet on the outer side ; and the teeth are 

 fixed to it by a bony mass occupying the place of their root, and incorporated 

 organically both with the tooth and with the jaw bone. The new teeth make 

 their first appearance, in cells, from within this osseous mass, and shoot irre- 

 gularly through its substance, gradually producing a necrosis in it, and thus 

 causing both the mass and the old tooth which it supports, to fall. 



Finally, the tooth in these genera becomes completely solid, its interior 

 cavity being filled up by the ossification of the pulpy substance ; so that they 

 do not appear to have many recurrences of fresh sets of teeth, and perhaps 

 have only one. 



The first two figures of Plate XV. represent the dentition of the croco- 

 dile ; the third, that of the fossil animal of Maestricht, which entirely agrees 

 with the ordinary lacertian type. Let us proceed to institute a comparison 

 between the teeth of these, and of the ichthyosaurus. 



1st, The teeth of the ichthyosaurus are lodged loosely in a long continuous 

 furrow, retained only, as it would appear, by the substance of the gum. This 

 structure is widely different from that of the monitors and ordinary lacertae, 

 where the teeth adhere to the jaw by a solid bony union. It differs much less 

 from that of the crocodile ; the only variation being, that the alveoli (which 

 in the crocodile are separate) here run together into one long continuous fur- 

 row ; — in which indeed the rudiments of a separation into distinct alveoli may 

 be traced, in the slight ridges extending, between the teeth, along the sides 

 and bottom of the furrow. (See PI. XV. fig. 12.) As corroborating this ana- 

 logy, I am informed that, in the crocodile, the three or four posterior alveoli 

 often run together into a continuous furrow, exactly in the same manner. 



