34 BLUE SHARK. 



loss; but as tlun-c is some degree of variety in the way in 

 which this process takes place in the different species, I will 

 enter a little farther into the description of it as I have 

 observed it in the present one. 



We have already seen that the seat of the tooth-formiiijr 

 process is in a thick membrane, which covers the jaws on their 

 inner surfiice, and which passes over them externally. This 

 membrane is in a condition of perpetual production, and at its 

 origin is formed into a series of cells or doublings, in each of 

 which the germ of a tooth may be discovered, soft and mem- 

 branous, and seemingly nourished from the sides of the sac or 

 cell itself. It lies fiat along the course of the membrane that 

 contains it, with the point directed downward in the lower jaw, 

 and towards the roof of the mouth in the upper jaw; in such 

 a manner as that in passing to its final destination, it has to 

 go through the third part of a circle, in the course of which 

 the upper doubling of the containing cell becomes torn through 

 its substance. The enamel of these teeth has no existence at 

 first; so that their substance is as soft and flexible as parchment; 

 but as their growth proceeds the nourishment from the sides 

 of the cell ceases, so that at last it is furnished only from the 

 root; and at this stage the circulation of nutriment by the 

 vessels appears to be from near the point, along the middle 

 line of each tooth, along which the solid firmness they at last 

 obtain is clearly to be discerned. The membrane within which 

 these teeth have been formed, is itself constituted of longitudinal 

 fibres, of some degree of firmness, with softer cellular membrane 

 at the part in wliich the teeth receive their actual formation; 

 and as in the course of nature, the former become more rigid 

 fi'om defect of nourishment, they contract in their substance, 

 and thus draw the roots of the teeth nearer to the situation 

 they are destined to occupy, but still leaving a vacancy Avhich 

 can only be supplied by the successive formation of teeth in 

 alternate order; the cells of one row being opposite to the 

 vacancies of the other, and only pressed closer, because 

 the fibrous membrane connecting them has in time admitted 

 of a more rigid contraction. In some species of this great 

 family, as the ■Nlonkfish, ( Squatiaa angelus,) and many of the 

 Ray tribe, the teeth cells are arranged in regular linear suc- 

 cession, without the filling up of the vacancies between them; 



