TEETH OF THE PORPOISE. 783 



principally the plate of whalebone : this, when got a certain length, 

 breaks off, leaving the hair projecting, becoming at the termination 

 very brittle : and the third, or intermediate * substance, by the time it 

 rises as high as the edge of the skin of the jaw, decays and softens away 

 like the old cuticle of the sole of the foot when steeped in water." 



(2259.) Other kinds of teeth, met with among Mammals, are com- 

 posed of calcareous earths deposited in a nidus of animal matter, and 

 consequently resemble bones in 



the hardness of their texture. Fig. 397. 



Tn their simplest form these 

 teeth consist of but one kind of 

 material, called ivory; and in 

 such cases there is no distinction 

 into classes as in the human 

 subject, every tooth being conical, 

 and formed upon a simple pulp. 

 Such are the teeth of the Por- 

 poises (Delphinidce) and of the Teeth of the Porpoise. 

 Cachalot Whales (Physeter}. The 



example selected to illustrate their structure and mode of growth is a 

 preparation of a portion of the jaw of the Bottle-nose Whale (Delphinus 

 tursio), contained in the Hunterian collection f. From this it is seen 

 (fig. 397) that each tooth of the Cetaceans in question is a hollow cone 

 of ivory (a, b, c, d), which, on being split longitudinally, is found to 

 contain a vascular pulp exactly filling up its internal cavity. It is 

 upon the surface of this pulp that the ivory matter is produced and 

 deposited, stratum inter stratum, within the tooth, thus gradually adding 

 to its substance as growth proceeds. In animals possessing a dental 

 apparatus of this description, Mr. Hunter observed that the teeth are 

 not at first developed in the jaw, but appear to form in the gum upon 

 the edge of the maxillary bones ; and that they either sink into the 

 jaw as they lengthen, or, as is more probably the case, the alveoli rise 

 to enclose their roots as growth advances. It would moreover appear 

 that these creatures do not shed their teeth, but that, as the jaw 

 enlarges, new teeth are constantly produced from behind, while those 

 towards the symphysis fall off, and their sockets become absorbed : 

 thus the size of the teeth is made to keep pace with the increasing 

 dimensions of the jaw J. The exact number of teeth met with in any 

 species of these Whales will evidently be uncertain. 



(2260.) In the male Narwal (Monodon) there are no teeth implanted 



* Mr. Hunter means by " intermediate," interposed between the contiguous 

 plates, not between the " hair " and the laminated whalebone. 



f Preps. Nos. 327 & 328. 



{ 'The Animal GEconomy,' by John Hunter, with Notes by Richard Owen, Esq., 

 F.R.S., p. 353. London, 1837. 



