52 CESTRACIONTS. 



disposition is associated therewith. The testaceous and crustaceous 

 invertebrate animals constitute, most, probably, the principal food of 

 the Cestracion, 



The teeth are attached to the maxillary surface by a slightly 

 contracted and truncated basis : the part which may be termed the 

 crown of the tooth is covered with a layer of dense white substance, 

 analogous to enamel, the surface of which is impressed by nume- 

 rous minute pits. Below the crown, the surface of the tooth is still 

 more irregular, and the basis is composed of coarse fibres with inter- 

 vening fissures and foramina. The small anterior teeth, which 

 resemble those of certain sharks, may be distinguished by their rugous 

 base, which is also broad and flat. 



When the dense outer layer is removed from the crown of the 

 newly formed teeth, the orifices of the medullary canals or tubes per- 

 forating the whole body of the tooth are brought into view. 

 The texture of this tooth, in fact, like that of the Myliobates, is 

 precisely such as would suggest the idea of the tubular texture 

 assigned by Cuvier to the compound teeth of fishes. These tubes 

 or canals, which are visible to the naked eye, are more or less 

 occupied in the recent fish by a vascular and organized medulla or 

 pulp. They are continued directly from the irregular and reticularly 

 disposed cells and canals of the semi-ossified cartilaginous crust of 

 the jaw. In the large crushing teeth, the greater number of the 

 medullary canals proceed in a pretty regular and slightly wavy course 

 towards the grinding surface, while the outer ones incline towards the 

 lateral surfaces ; but they soon begin to divide, and the divisions 

 continue to ramify dichotomously ; the branches anastomose together, 

 especially near the surface, and form loops of which the convexity 

 is always directed but flattened, towards the unattached surface of 

 the tooth. The medullary canals are, in general, slightly dilated 

 before they dichotomize, and the branches maintain throughout nearly 

 the same size as the trunk. 



Plate 12 shows the general appearance of the medullary canals 

 in a vertical section, including part of the lateral surface of the tooth, 

 as seen by transrtiitted light through a compound lens of half an inch 

 focus. The dark colour of the wide medullary canals is due to the 



