8 ATTACHMENT. SUBSTANCE. 



inwardly projecting convex ridge. The masonry of this internal 

 buttress and of the dome itself, is composed of hollow columns, every 

 one of which is placed so as best to resist or transmit in the due 

 direction the superincumbent pressure. The advantages gained by 

 this beautiful example of animal mechanics will be explained when the 

 dental system of the labroid fishes is described. 



In another case, in which long and powerful piercing and lace- 

 rating teeth were evidently destined, from the strength of the jaws, 

 to master the death-struggles of a resisting prey, we find the broad 

 base of the tooth divided into a number of long and slender cylindrical 

 processes, which are implanted, like piles, in the coarse osseous sub- 

 stance of the jaw ; they diverge as they descend, and their extremities 

 bend and subdivide like the roots of a tree, and are ultimately lost 

 in the bony tissue. This mode of implantation of the teeth, which 

 I have detected in a large extinct sauroid fish {Rhizodus) ,{l) is, per- 

 haps, the most complicated wliich has yet been observed in the animal 

 kingdom. 



6. Substance. — The teeth of fishes, in respect to their sub- 

 stance, present various degrees of density and complexity. In most 

 of the chsetodonts they are flexible and elastic, of a yellowish, shining, 

 and subtransparent tissue. The labial teeth of the helostome are 

 also of this kind, as are also the anterior maxillary teeth of the gonyo- 

 donts, and of the percoid species, hence called Trichodon. In the cyclos- 

 tomes, the teeth consist of an albuminous tissue, of a somewhat denser 

 nature. The upper pharyngeal molar of the carp, consists of a peculiar 

 brown and semi-transparent tissue, harder than the true horny teeth 

 of the lamprey. 



The greater number of fishes have their teeth composed of an 

 osseous substance, somewhat denser than the jaws to which they 

 are affixed. In some instances, as in the teeth of the flying-fish 

 (Exoccetus) , and sucking-fish (Remora) , the substance of the tooth is 

 uniform, and not covered by a layer of a denser texture. In others, as 

 the shark, sphyraena, &c., the tooth is coated with a dense, shining, 

 enamel-like substance ; but this is not true enamel, nor the product 

 of a distinct organ ; it differs from the body of the tooth only in the 

 greater proportion of the earthy particles, their more minute diffusion 



(1) PI. 36. 



