390 NATURAL SCIENCE. June, 



teeth, but in the Teleostei, where these bones are more deeply placed, 

 the teeth are more restricted in their distribution. It is only in the 

 most lowly organised forms, e.g., Alhida, that a more uniform distri- 

 bution of teeth, at all approaching that which occurs in the Ganoidei, 

 is to be observed. Teeth are very extensively developed on the 

 palatine bones in the pike [Esox), but in a great number of Teleostei, 

 e.g., the cod (Gadus), the palatines are edentulous. The exceptional 

 dentitions of Diodon and Scanis have already been alluded to (p. 387). 

 The dentition of Tetrodon is essentially similar to that of Diodon, except 

 that only the marginal set of tooth-plates is present. Triodon, on the 

 other hand, appears to be more related to the Scaroids, judging by the 

 nature of its dental armature. In the file-fish (Balistes) the teeth are 

 relatively large and are disposed in a double row in the premaxilla, 

 and a single row in the dentary. In Thyrsites and SphyrcBua the teeth 

 at the front of the premaxilla are very large in size and are back- 

 wardly directed, in a manner recalling the disposition of the poison 

 fangs in a venomous snake. 



Gill-teeth, developed on the pharyngeal margins of the branchial 

 arches, are usually small in size and thickly set {e.g., pike). They 

 may be united by their bases into small plates, but the plates are 

 merely embedded in the mucous membrane, and are not united to 

 the bone. In the sun-fish (Orthagoriscus) the gill-teeth are enormously 

 developed. They are elongated and sharply pointed and are anchy- 

 losed to the branchial skeleton. The carp {Cypvimis) is edentulous 

 so far as the mouth proper is concerned, but the two lower pharyngeal 

 bones carry well-developed teeth, opposed to a callous tubercle 

 projecting from the basioccipital bone. It may be taken as a general 

 rule that the pharyngeal dentition is inversely proportional to the 

 extent of tooth development on the jaws. In the wrasse (Labrus) the 

 upper pharyngeal bones bear teeth as well as the lower. The teeth 

 are of hemispherical shape and have a vertical succession (Fig. 20). 

 The pharyngeal teeth of the extinct Phyllodus have the form of 

 flattened, slightly convex plates. Underlying each of these is a series 

 of five or six similar plates of the nature of reserve teeth. The older 

 plates are not shed as a whole, but wear away in places, leaving the 

 younger replacing teeth partially exposed. The pharyngeal teeth of 

 the parrot-fish [Scams) are compressed antero-posteriorly and are 

 developed in common alveolar cavities (Figs. 21 and 22, s) situated 

 at the posterior end of the lower and at the anterior end of the upper 

 pharyngeal bones. The upper teeth therefore succeed one another in 

 a backward, and the lower in a forward direction. The teeth are 

 developed independently, but are firmly anchylosed to the bone and 

 to one another before they come into active use. There is a close 

 analogy, as Owen points out (4, p. 381), between the dental mass 

 carried by the pharyngeal bones of Scams and the complicated molar 

 teeth of the elephant, both in form, structure, and in the reproduction 

 of the component denticles in horizontal succession. 



