12 STRUCTURE. 



interspaces are occupied by the calcigerous tubes and cells. The me- 

 dullary canals are directly continued from those of the common bone 

 with which the base of the tooth is anchylosed, or into which it has 

 been converted. As the medullary canals proceed through the tooth, 

 they maintain a course more or less parallel, and more or less straight, 

 or wavy ; but they ramify abundantly, and gradually diminish in 

 calibre as they approach the surface of the tooth. The illustra- 

 tions of this modification of the dental structure in the present work, 

 are taken from the teeth of the extinct Lamna, Didyodus, and Sauroce- 

 phalus, and from those of the recent Sphyroena and Acanthurus. In 

 the latter genus the dendritic arrangement of the medullary tubes 

 recognized by Mr. Andre, has subsequently been figured by V. Born, 

 V. Born and Retzius have described a similar structure in the teeth 

 of the wolf- fish {Anarrhichas) , of which Mr. Nasmyth has given a 

 figure in his useful translation of some of the recent treatises on 

 dental anatomy. 



The reticulate medullary tubes pervade the structure of the 

 teeth of the percoid, scieenoid, cottoid, and gobioid families of fishes ; 

 and of those of the Capros, Naseus, and other genera of the Theuties of 

 Cuvier, besides the teeth of the Acanthuri already cited. A similar 

 reticulate structure is common to the teeth of the Chcetodontes and the 

 Pleuronectes : in the cycloid fishes, we find it almost universal in the 

 scomberoid, lucioid, salmonoid, and clupeoid families : it is ex- 

 changed for a higher type of structure in the maxillary teeth of the 

 lophioid fishes, and in the pharyngeal teeth of the cyprinoids, but 

 it again reappears in the teeth of the blennioid, gadoid, and mu- 

 raenoid families ; and the same coarse bone-like structure pervades 

 the dental plates of the supposed amphibious Lepidosiren. 



The higher type of structure just alluded to is that which 

 characterises the teeth of most reptiles and mammalia. Here the 

 dentine consists of a single medullary or pulp canal, and a single 

 system of calcigerous tubes radiating from the central or sub-central 

 canal, at right angles to the periphery of the tooth. The teeth 

 of the extinct sauroid fishes and pycnodonts, the maxillary teeth of 

 the existing file-fishes {Balisies), and angler (Lophius), the incisors, 

 canines, and molars of the breams or sparoid fishes, the pharyngeal 

 pavement-teeth of the wrasse-tribe (Labridce), the maxillary and 



