THE ELASMOBRANCII FISHES 



35 



Saw Tooth 



A secondary form of hypertrophy is seen in the saw tooth of the sawfishes. 

 Here the teeth are arranged along both edges of the saw (rostrum) as greatly 

 modified scales. In the specimen from which figure 38 was taken the teeth 

 were asymmetrical, 26 teeth being i)resent on the left edge of the saw and 27 

 on the right. In the larger saws, some of which may reach a foot in width and 

 six feet in length, the crown of the tooth may reach four inches in length and 

 the teeth become most formidable organs of offense. 



A sagittal section through an adult saw tooth according to 

 Engel (1910) shows that the core of the tooth, unlike that of the 

 spine, becomes converted into long columns of vasodentine, and 

 a transverse section through this dentine near the tip also dem- 

 onstrates numerous canals around which the dentine is formed 

 and through which l^lood vessels pass (see section through tooth, 

 p. 131, fig. 129). 



A saw tooth erupting from the side of the rostrum (fig. 41) 

 presents something of the appearance of a developing fin spine 

 of Acanihias, but with one difference. The saw tooth arises di- 

 rectl}^ through the mass of invaginated epidermis {ep.) and 

 hence the posterior part is more completely surrounded by an 

 enamel organ {e.o.). It follows that there is a layer of enamel 

 over that part of the saw tooth just as there is over the placoid 

 scale. 



A transverse section through the saw tooth near its tip (fig. 

 39) shows that, like the fin spine of Acanthias, it is more or less 

 flattened posteriorly. The central part of the immature tooth is 

 occupied by the enlarged pulp cavity containing blood vessels 

 and numerous odontoblasts. Just outside of the pulp cavity is 

 the layer of dentine {d.'), superficial to which is the thinner 

 band of enamel or vitrodentine (e.). The tissues outside of the enamel organ 

 {e.o., fig. 41) do not take part in the production of the saw tooth. They are to 

 be looked upon as the intermediate and superficial layers of the epidermis. 



Fig. 38. 



Saw tooth 



of Pristis 



antiquorum. 



Sting 



A third form of hypertrophy occurs as the variously formed defensive organs 

 of the sting rays (fig. 42) . Here a spine arising from the dorsal part of the tail 

 grows backward, varying greatly in size and in complexity. In the small sting 

 ray Urolophus this spine is but two inches in length; while in the larger ty]ies. 

 as in Myliohatis, it may reach a length of four or five inches. Usually the sting 

 or spine is simple, but it may be compound, numerous spines arising one be- 

 hind the other. Simple or compound, each spine is provided with a sharp 

 point, and its sides have smaller and recurved hooks arising from them. 



