GYMNODONTS. 81 



The exposed margins of the upper and lower jaws of the Diodon, 

 which appear to be covered with a thick irregular layer of the same 

 dense white dental substance as that of the posterior triturating 

 masses above described, owe their apparently simple character to a 

 still more complicated structure. They consist of a series of narrow 

 flattened denticles lying horizontally, and at right angles to the an- 

 terior surface of the jaw ; so that those of the exterior row of one 

 jaw have their sides opposed to the corresponding row in the 

 opposite jaw when the mouth is closed. These denticles are deve- 

 loped in a cavity, between the outer and inner walls of the jaws, 

 the floor of which is formed by a thin cribriform osseous plate, sepa- 

 rating the cavity containing the teeth (PL 38, fig. 2, b) from the wide 

 vascular canal fcj which occupies the substance of the jaw. At the 

 bottom of the dental cavity, the denticles are seen in different stages 

 of formation, unattached, but closely packed, and with their lateral 

 margins overlapping each other. They are of an oval form, with 

 the under surface, or that next the floor of the cavity, slightly con- 

 cave and smooth : the opposite surface convex, and honeycombed : 

 the denticles gradually diminish in size from the middle to the 

 outer ends of the dentated margin. As their formation reaches its 

 completion, the denticles become anchylosed to each other and to 

 the osseous parietes of their cavity by ossification of the capsules of 

 the calcified pulps. The thin layer of bony matter or cement en- 

 closing the denticles is soon worn away when they reach the margins 

 of the jaws, the irregularity of which is caused by the alternately 

 overlapping arrangement of the exposed denticles. 



The order of development and succession is the same in the mar- 

 ginal as in the posterior teeth, they ascend in the lower and des- 

 cend in the upper jaw, but are pushed on in the horizontal instead 

 of the vertical direction. The chief difierence is, that whereas in the 

 posterior dental tubercle there are only two broad lamelliform teeth 

 in the same plane, in 'the marginal series there are upwards of forty 

 narrower denticles. 



Cuvier observes that the Tetrodons differ from the Diodons inasmuch 

 as they have no posterior triturating disk, but only the marginal 

 plates ; and have the jaws divided, each into two portions by a den- 



