CH^TODONTS. 105 



base of the tooth, and are occupied, not by blood-vessels only, but 

 by processes of the organized medulla or pulp, including the nutrient 

 vessels, nerves, and connecting tissue. The diameter of the medullary 

 canals at their origin is i^th of an inch, and their interspaces equal 

 from four to six of these diameters ; their general course is parallel 

 to each other and to the axis of the tooth ; but they begin to send 

 off side-branches soon after their origin, and continue to ramify 

 abundantly throughout their course. The ramifications anastomose 

 and form arches, of which the convexity is directed towards the 

 margin of the tooth. Both the main canals and their branches 

 gradually diminish in size as they approach the margins. The cal- 

 cigerous tubes, which are every where continued from the medullary 

 canals, are of extreme tenuity ; the marginal ones, which have a 

 direct course perpendicular to the surface of the tooth, do not 

 exceed aaUooth of an inch in diameter. A diminished view of four of 

 the teeth of the Acanthurus nigricans, as seen with a magnifying 

 power of sixty linear dimensions, is given in PL 44, fig. 1. The 

 imbricated disposition of the teeth is here shewn. 



CHjETODONTS.(I) 



45. The name of this family of fishes is significative of the peculiar 

 form of the maxillary teeth, which resemble the hairs of a fine brush, 

 and are of a soft subtransparent flexible texture, in the majority of 

 the genera composing it. The pulp-cavity is continued to near the 

 apex of these slender elongated denticles. (2) The true Chsetodonts 

 thus characterized, have no teeth on the palatine, vomerine or lingual 

 bones. 



In the genus Chcetodon proper, the setiform teeth are longer than 

 in the genera Heniochus or Phelmony in the latter of which they 

 resemble rather the pile of velvet. In the genus Zanclus the setiform 

 maxillary teeth are directed forwards. 



In the Holacanthus, the setiform teeth are finely pointed ; but in 

 Platax,{3) the outer row have their extremities compressed, slightly 

 expanded and terminated by three pointed lobes. 



(1) xa^T'l, a bristle ; o^«Ci a tooth. PL 1, fig. 2 (2) PI. 4'i, fig. 3, a and b. (3) PI. 1, fig. 2, a. 



