DIGESTIVE SYSTEM. 233 



The genera Ceratodus and Ctenodus have two teeth ab ve and 

 two below, while in Chimaera we find four superior and two inferior 

 teeth. 



The substance of which the teeth in Fishes is composed presents 

 very different characters. The teeth of the Cyclostomi belong to 

 the category of epithelial structures, or horn-tissue. The dental 

 plate upon the occipital process of the Carp consists of a peculiar 

 brown, semitransparent tissue, harder than the substance of the 

 horny teeth of the Lampreys. In most Chaetodonts the teeth are 

 delicate, flexible, and elastic, and composed of a yellow transparent 

 tissue, this being the case also in the labial teeth of Helostomus. In 

 most Fishes, however, the teeth consist of Osseous substance, 

 slightly denser than that of the jaws to which they are attached. Oc- 

 casionally, as in Exocoetus and Echeneis, the substance of the teeth 

 is uniform throughout, and not invested by a layer of denser texture. 

 In others, c. g., the Sharks, the tooth is covered by a dense trans- 

 parent enamelloid substance ; it is not, however, true enamel, but 

 the proportion this substance bears to the rest of the tooth may be 

 determined from the larger quantity of earthy constituents which 

 it contains, the finer division and more parallel arrangement of its 

 calciferous tubules. In Sargus and Balistes the peculiar osseous 

 substance of the tooth is still harder, and covered by a thick layer 

 of a denser substance, which differs little from the enamel of the 

 higher Vertebrata. In Balistes also, and some other Fishes, we 

 find a third layer superadded, that may be compared to the cementum 

 of the Mammalia. In Scarus there is found even a fourth material, 

 forming a very dense ivory-like layer upon the periphery of the 

 tooth. In Fishes the small dental tubes (canaliculi chalcophori) are 

 extremely distinct, and the microscopic structure of the teeth in 

 general exhibits manifold modifications, which can not be further 

 discussed in a work like the present. 



The Intestinal Canal offers for consideration a number of diversi- 

 ties, which, like those of the teeth, can not be well described without 

 entering into the anatomy of the several orders and genera. 



The Cavity of the Mouth in the Osseous Fishes opens externally 

 upon either side through the branchial fissures. Peculiar tooth-like 

 processes stand out upon the internal edge of the branchial arches, 

 and protect the fissures between the latter from the intrusion of 

 food. The pharynx commences in the Osseous Fishes immediately 

 behind the pharyngeal teeth, and there is found surrounding it in 

 this situation a strong sphincter-like muscle; an cesophagus may be 



