! 



CESTRACIONTS. 49 



side of the tooth is begun. These facts are strikingly opposed to the 

 theory of the formation of a tooth by the transudation of layers from 

 the superficies of a pre-existing glandular pulp. 



As the teeth of the Myliobates are gradually carried forwards into 

 action by the direction of growth of their basis of support, the arese of 

 the medullary canals become progressively diminished, as in bone, by 

 osseous deposition in concentric layers, and are thus finally consoli- 

 dated in the anterior teeth. 



CESTRACIONTS. 



18. The dental characters of this family of cartilaginous fishes, are 

 chiefly manifested in a form of tooth, better adapted for crushing or 

 comminuting alimentary substances which offer only passive resistance, 

 than for piercing, cutting and lacerating a living prey ; and this less 

 formidable character of the maxillary armour is compensated, in gene- 

 ral, by the development of two formidable spines upon the back of 

 the fish. In most of the species, the teeth also vary in form and 

 size in the same individual to a greater degree than in the sharks ; 

 and in all the cestracionts, their substance is traversed by medullary 

 canals whose systems of calcigerous tubes are not separated by well 

 defined boundaries, as in the Myliobates. 



Of the numerous singular forms of this tribe of cartilagi- 

 nous fishes that once peopled the seas of the northern hemi- 

 sphere, and which have left their less perishable remains in the 

 secondary strata of the present dry land, all have now disappeared, 

 and the sole existing representative is the genus Cestracion, 

 of which the most common species is met with in the Australian 

 Seas : a second species has been indicated which frequents the 

 southern coasts of China. The ancient fossils above alluded to 

 would have been scarcely intelligible unless the key to their nature 

 had been afforded by the teeth and spines of the existing Cestracion. 



In the Port Jackson shark, {Cestracion Phillippii), the jaws form 

 a greater proportion of the skull than in any other existing carti- 

 laginous and plagiostomous fish ;(1) they are also more elongated and 

 directed more horizontally forwards, thus approaching nearer to the 



(1) PL 10, fig. 1. 



£ 



