220 A MANUAL OF DENTAL ANATOMY. 



not possibly be distinguished from teeth, the resemblance 

 both in outer form, in minute structure, and manner of 

 development being most complete. The tooth figured on 

 page 90 is a fair example of a structure very common among 

 the sharks, viz., a central body of osteo-dentine, the outer 

 portion of which has dentinal tubes so fine, regular, and 

 closely packed as to merit the name of hard unvascular 

 dentine, and over this again a thin varnish of enamel. (?) 



And yet no observer from its structure alone could feel 

 .sure whether it was a large dermal spine, or a tooth. Dental 

 tissues occur in other parts of the mouths of Selachia than 

 upon the jaws, not only in the embryonic stages, but in the 

 .adult. Thus Professor Turner has described (Proc. Roy. 

 Society, Edinburgh, 1880), very numerous comb-like ap- 

 pendages 5 inches long upon the branchial arches of the 

 Basking Shark (Selache maxima), which apparently perform 

 the same function as whalebone in straining the water. 

 These combs are formed of a variety of dentine (? osteo- 

 dentine), and closely resemble in structure the true teeth, 

 which are however very small in this shark. 



In the seas of Australia there exists a Shark, the 

 Cestracion Philippi, with a very aberrant dentition, to which 

 great interest attaches, inasmuch as it is the sole surviving 

 representative of forms once spread all over the world. In 

 the front of the mouth the teeth are small and very 

 numerous ; they are flat plates fitted by their edges to one 

 another, while from their centres spring up sharp points, 

 soon worn off when the tooth reaches such a position upon 

 the jaw that it comes into use. 



Proceeding backwards, the teeth cease to be pointed, 

 increase in size, and become fewer in each row ; a reference 

 to the figure will convey a better idea of their general form 

 than any description. Those which have come into use 

 are, towards the back of the mouth, always much worn ; 

 their shedding and renewal takes places, as in other sharks, 



