GYMNODONTS. 79 



endo-skeleton of the Diodon. The main tubes of the dental plate are 

 continued immediately from the cells of the osseous septum ; they 

 proceed for a short distance vertically, or with a slight curvature, 

 in the substance of the dental plate, and then quickly divide and 

 subdivide, the branches generally coming off at an angle of 45°, being 

 slightly bent, crossing each other in an inextricable manner, and ter- 

 minating ultimately in the clear matrix of the upper surface of 

 the dental plate. 



Each dental lamella presents, at every part, the same organized 

 structure which is above described and delineated ; there is no part 

 which offers the crystalline characters of true or mammalian enamel. 

 The mucous membrane of the mouth and periosteum of the jaws, are 

 reflected into the cavities at the base of the compound tooth (PL 38, 

 fig. 2, a) ; the periosteum lines the parietes of the cavity, and the mu- 

 cous membrane forms a thick cushion, extended across its floor. From 

 this surface, a lamelliform pulp is developed, in which the calcifying 

 process takes place in a direction from above downwards : at first, 

 the earthy salts are deposited in the state of such minute subdivision 

 and in such a direction and abundance as to produce the dense and 

 minutely tubular structure of the dental plate ; when this has acquired 

 its due thickness, the rest of the pulp becomes ossified, i.e. the cal- 

 careous salts are deposited in less abundance, and in the parietes 

 and interspaces of coarse cells, instead of those of minute tubes. The 

 margins of the ossified pulps, by this process, become confluent with 

 the parietes of the general dental cavity, and the mutual adhesion 

 of the flattened surfaces of the impacted lamelliform teeth is promoted 

 by the pressure to which their exposed surfaces is subject. By the 

 time that ossification has begun in one pulp, a second has been 

 developed beneath it, and it is the portion of the pulp solidified by 

 the fine tubular calcification which gives rise in the macerated and 

 dried jaws to the loose and thin lamellae in the dental cavity. These 

 lamellae become fixed by means of the coarser calcification or ossi- 

 fication which subsequently takes place in the remains of the pulp, 

 and their margins are thus anchylosed to the surrounding bone, in 

 a manner analogous to the fixation of the base of the ordinary 

 shaped teeth in other fishes. 



