32 SHARKS. 



In the genus Mustelus, to which our " smooth dog-fish " 

 belongs, the teeth deviate from the form typical of the sharks and 

 approach those of the rays, being obtuse and rounded, with fine trans- 

 verse ridges, without a piercing cusp or cutting edge, but adapted for 

 bruising and crushing : they are numerous, small, nearly equal, 

 arranged like a pavement in the quincuncial order. 



The maxillary teeth of the saw-fish fPristisJ resemble those of the 

 smooth dog-fish, but as the more fornaidable part of the dental system 

 of this genus offers some peculiar characters, it will be subsequently 

 described. 



In the Cestracion both prehensile and crushing teeth are asso- 

 ciated together in the same jaws, but as we here meet with a modifi- 

 cation of the microscopic texture of the teeth different from that 

 of the teeth in the true sharks, a separate section will be devoted to 

 their description, and to that of the teeth of numerous cognate fossil 

 species of plagiostomes. 



In all the genera of sharks, the body of the tooth is principally 

 occupied with the two kinds of canals which I have termed " me- 

 dullary" and " calcigerous."(l) The latter are, however, essentially 

 minute branches or continuations of the former, and although, in 

 the newly formed tooth distinguishable by the nature of their contents, 

 yet this difference is gradually obliterated by the progressive depo- 

 sition of calcareous matter by concentric layers in the medullary 

 canals. 



In plate 6, a view of the structure of the tooth of an extinct species 

 of shark {Lamna elegans, Ag.) is given as seen in a thin longitudinal 

 section under a compound lens of one inch focus. With this power the 

 medullary canals alone are visible, the minute calcigerous tubes giving 

 rise to the cloudy appearance in their interspaces. The medullary 

 canals are continued from the short and small pulp-cavity at the base 

 of the tooth. The principal branches run parallel with the axis of the 

 tooth, but quickly give off ramuli, which are directed transversely, 

 and again ramify at right angles or nearly so, and anastomose, so as 

 to form a beautiful reticulate arrangement of tubes, very similar to a 

 network of capillary vessels, throughout the whole substance of the 



(.1) Report of the British Association, 1838, vol. vii, p. 135. 



