SHARKS. 41 



and a series of deep alveoli is excavated in each of its lateral mar- 

 gins. 



The teeth which are lodged in these sockets are elongated, com- 

 pressed in the same plane as that of the body of the saw, and the mar- 

 gins converge to a sharp point, which is situated a little behind the 

 axis of the tooth ; the anterior border of the tooth is convex, but grows 

 sharper towards the point ; the posterior margin is concave or grooved, 

 and the groove glides upon a corresponding ridge which projects into 

 the back part of the socket. The rostral tooth is solid, as shewn in 

 the longitudinal section of the one figured in PI. 8, fig. 3 ; its base, 

 (fig. 5,) is slightly concave and porous, hke the section of a cane, but 

 the pores are finer and more numerous. The walls of the socket are 

 formed by ossification of the rostral cartilage to the adequate extent, 

 {b. fig. 3) ; but as undue weight, under any circumstances, and espe- 

 cially at the fore end of the fish would be a cumbrous impediment to 

 its motions, the intervening spaces {d. fig. 3) between the sockets 

 are hollow and filled with a gelatinous medulla. A large vascular 

 canal (c. fig. 3) traversed by branches of the facial artery, and of the 

 second division of the fifth pair of nerves, and inclosed in a cellular 

 and gelatinous tissue, runs parallel with the axis of the saw along the 

 back part of the alveoli, and supplies the materials for the increase 

 of the teeth, which are not shed and renewed like^" the maxillary teeth, 

 but grow with the growth of the body by constant addition of fresh 

 pulp-material progressively ossified at their base.(l) 



The structure of the rostral teeth of Pristis has the nearest re- 

 semblance to that of the maxillary dental plates of Myliohates ; they 

 are traversed throughout by medullary canals which run parallel to 

 each other and to the axis of the tooth ; but they exhibit more fre- 

 quent anastomoses and dichotomous sub-divisions than in Myliohates. 

 The diameter of the medullary canals is ^th inch, and their inter- 

 spaces Ath of an inch near the base of the tooth ; they are sur- 

 rounded by concentric laminae which increase in number as the 



(1) In the Pristis cuspidatus the rostral teeth are broad, and lancet-shaped ; in the Pristis 

 microdon they are very short ; in the Pristis cirratus the rostral teeth vary in length, there being 

 from three to five smaller ones interposed between the longer teeth, which are sharp-pointed and 

 slightly recurved. The base of these teeth expands, and is excavated more deeply than in the 

 common saw-fish ; it is not socketed but anchylosed to the margin of the rostrum. ITie rostral 

 teeth in this species extend along the sides of the head beyond the angles of the mouth. 



