SUBSTANCE. CHEMICAL COMPOSITION. 9 



through the gelatinous basis, and the more parallel arrangement of 

 the calcigerous tubes ; but it is developed in and by the same matrix, 

 and, resulting from the calcification of its external layer, is the first 

 part of the tooth which is formed. In the Sargus and Balistes, the 

 dentine, or proper osseous substance of the tooth, is harder than that 

 of the fishes last cited, and is covered with a thick layer of a denser sub- 

 stance, developed by a distinct organ, and differing from the enamel of 

 the higher animals only in the more complicated and organized mode of 

 deposition of the earthy particles. The ossification of the capsule of the 

 matrix gives the enamel of the teeth of the file -fish, and some others, 

 a thin coating of a third substance analogous to the ** csementum, 

 or crusta petrosa," of the mammaUan teeth. And in the pharyngeal 

 teeth of the parrot-fish, a fourth substance is added to the structure 

 of the tooth by the coarser ossification of the pulp, after its peripheral 

 portion has been converted into the dense ivory. The teeth, thus 

 consisting of dentine, enamel, cement, and coarse bone, are the most 

 complicated as regards their substance that have yet been discovered. 

 7. Chemical composition. — With respect to the chemical compo- 

 sition of the teeth of fishes, little remains to be added to what has been 

 stated on this subject in the preliminary general observations. The 

 animal base of the horny teeth of the cyclostomes is albuminous, as 

 in true horn ; that of the calcified teeth is gelatinous, and the pro- 

 portion of gelatin to the usual hardening salts, diminishes as their 

 density increases. According to the analysis of Lassaigne,(l) the teeth 

 of the shark yield 



100, 



The inferior pharyngeal teeth of the carp contain 



Phosphate of lime . . 49 

 Carbonate of lime . . 16 

 Animal matter ... 35 



100,0 

 (1) Berzelius, Traite de Chimie, par Esslinger 1828, torn, vii, p. 480. 



