June, 1896. THE TEETH OF FISHES. 381 



In the teeth of the eagle-ray {Myliobatis) and in the rostral teeth 

 of the saw-fish (Pristis), the pulp is broken up into a great number of 

 delicate parallel columns, around which the dentine is regularly- 

 deposited. A transverse section through the middle of such a tooth 

 exhibits numerous radiating dentinal systems, each about what 

 appears to be a separate pulp ; but a section taken nearer the base 

 shows how these pulp-columns unite below into a common pulp-mass. 1 



Besides this hard dentine, traversed by fine protoplasmic fibrils, 

 there is another kind, termed vasodentine, in which dentinal tubules 

 cannot be distinguished, but through which numerous irregular blood- 

 capillaries ramify (Fig. 1, vd.). The minute structure of this dentine 

 is much coarser than that of the former (Fig. 2, d.), and when, as 

 frequently happens in fishes, the two kinds occur in the same tooth, 

 it is the vasodentine which is internal and in closer relation with the 

 pulp. A third kind, called osteodentine, bears a closer resemblance to 

 bone than the two foregoing varieties of dentine. It is not developed 

 entirely on the surface of the pulp, and is not related to an odonto- 

 blast layer, but is deposited irregularly within the pulp. The channels 

 and spaces of this form of tooth-tissue, therefore, differ from those 

 of vasodentine, not only in their superior size, but in being occupied 

 by pulp and not by capillary blood-vessels only (Fig. 2, od.). 



The prismatic structure of the enamel is not well marked in the 

 teeth of fishes, and this accounts for the relatively small number of 

 cases in which its presence has been recognised. It is undoubtedly 

 absent in some cases, e.g., Labvus. Cement is of rare occurrence in 

 fishes. A coarse form of cement, called by Tomes (9) ' bone of 

 attachment,' serves in many cases to fix the tooth to the jaw ; but 

 since this substance is said to be developed from the bone of the jaw, 

 and not from the tooth, it can hardly be regarded as the exact 

 equivalent of the cement or ' crusta petrosa ' which attains so great 

 a development in the teeth of some mammals. 



The teeth of Cyclostomata are epidermal in origin, and consist of 

 hollow cones of a horny material belonging to the stratum comeum of 

 the skin. In the hag-fish (Myxine), and in Bdcllostoma, Beard (1) has 

 shown that the dermal papilla develops an imperfect calcified tooth 

 beneath the horny epidermal tooth. 



Attachment. — Although teeth are structures developed entirely 

 and solely from the mucous membrane covering the jaws, it is very 

 commonly found that, in order to obtain that support which is 

 necessary for the due performance of their function, the teeth acquire 

 an intimate connection with the underlying skeletal parts. So 

 accustomed are we, in fact, to this intimate relationship between 

 tooth and bone, that we have come to regard the teeth as an essential 

 of the jaw; and a skull from which the teeth have been lost we 

 discard as imperfect. In the sharks and rays the teeth retain their 



1 A similar structure is presented, among Mammalia, in the teeth of the Cape 

 ant-bear, Orycteropus. 



