14 STRUCTURE. 



fishes, especially the Scarus and Diodon, which have been cited as ex- 

 amples of the fourth modification of the dental structure. These teeth 

 strike fire with steel, yet they present an organized structure of mi- 

 nute complexity, and the calcigerous tubes are nowhere so numerous, 

 so minute, so beautifully ramified and interlaced together. 



It has already been observed that the prismatic maxillary denticles 

 of the Scarus, being compacted together side by side and with their 

 single medullary canals parallel to each other, produce a compound 

 dental plate analogous to those which were first cited in illustration of 

 the present subject. In the Diodon, each denticle, which is composed 

 as in the Scarus of a single system of minute calcigerous tubes, assumes 

 the form of a thin plate : the pulp cavity instead of being contracted 

 into a tube, as in the elongated teeth, is here spread over the under 

 surface of the dental lamella ; the calcigerous tubes proceed in a di- 

 rection more or less vertical to the upper surface (in the teeth of the 

 lower jaw); and the compound dental mass results from the super- 

 position and successive development of similar plates, separated 

 only by a layer of thin bone or csementum. In the pharyngeal teeth 

 of the Scarus, the denticles are also more or less lamelliform, but their 

 position is vertical, and they are joined side to side by means of the 

 intervening cement or the ossified capsule : compound dental masses 

 similarly constructed are present in the capybara, elephant, and 

 others of the mammalia, generally cited as affording examples of the 

 most complex teeth. 



9. Development. — The teeth of fishes are formed according to the 

 general laws of dental development already discussed ;(1) but the pro- 

 cess, in many instances, does not extend beyond the earlier and simpler 

 stages observable in the higher classes of animals. In all fishes, as 

 in other vertebrate animals, the first step is the production of a simple 

 papilla from the free surface of either the soft external integument, 

 as in the young Pristis,(2) or of the mucous membrane of the mouth, as 



(1) Introduction, p. vi. 



(2) A very close analogy exists between the dermal bony tubercles and spines of the cartilagi- 

 nous fishes and their teeth. The thick enamelled scales of the ganoid fishes of Agassiz exhibit 

 an organization similar to that of the teeth : the system of minute parallel tubes, with their 

 branches and anastomoses, in the thick scales of the extinct lepidotus, is as complicated as in 



