SHARKS. 39 



As a consequence of a formation of a tooth by conversion of, 

 instead of transudation from a pre-existing pulp, the successive for- 

 mation of these pulps necessarily follows, where a succession of teeth 

 is required ; these reproductive pulps are developed in the shark in 

 the vascular mucous membrane at the angle of reflection of the 

 thecal fold upon the groove at the basal line of the jaws. They 

 gradually advance from this situation towards the margin of the jaw, 

 the centripetal ossification extends as they advance, and consolidation 

 is completed by the time they are ready to change their recumbent 

 for the erect position, and take the place of the tooth previously 

 shed. 



This change of place and direction is well known to be not the 

 effect of muscular contraction, but of partial absorption and deposi- 

 tion operating upon the membrane to which the teeth are attached. 

 This membrane is gradually brought to the exterior of the jaw, and is 

 then removed together with the attached tooth, supposing the latter 

 not to have been already violently displaced. But the following 

 question now offers itself: — Does this movement of growth take place 

 simultaneously in the membrane and the jaw to which it is attached, 

 or is it a slow and gradual sliding motion of the dentigerous membrane 

 upon the jaw ? 



To determine this question would require an experiment similar 

 to those by which Duhamel and Hunter traced the change of place 

 in the particles of growing bone ; a foreign body e. g. should be in- 

 serted into the base of the jaw of the shark, and a tooth in the corres- 

 ponding place should be so marked, as at a subsequent period it might 

 be recognized, and its position compared with the perforated part of 

 the jaw. Such an experiment would not be very practicable in the 

 carnivorous inhabitants of the deep we are now considering, but acci- 

 dent has satisfactorily supplied its place. The jaws of a large 

 Galeus passed into the private collection of an English anatomist, 

 in which the barbed spine of a sting-ray {Trygon) had been driven, 

 during a predatory attack of the shark, into the lower jaw through 



matrix of animal matter ; but they diifer as to the direction of the deposition, which in bone is 

 from the centre to the circumference, in tooth from the circumference to the centre ; the pro- 

 gress of calcification in the one is centrifugal, in the other centripetal. — See Comptes Rendus 

 de I'Academie dcs Sciences, 1839,^. 784. 



