SITUATION. 



latines, (b b PL 61), the median one is the vomer, (c PL 61). Two flat- 

 tened bones on each side, called the pterygoid and transverse bones, 

 complete the bony arch or buttress which extends from the inter- 

 maxillaries to the pedicle supporting the lower jaw. Posteriorly, the 

 roof of the mouth is completed by the sphenoid and sub-occipital bones. 



The floor of the mouth is supported by the median longitudinal 

 chain of lingual bones, to the sides of which are attached the infe- 

 rior extremities of the branchial arches ; these form the sides of the 

 posterior part of the mouth, which gradually contracts to the pharynx ; 

 this orifice is strengthened by bones above and below, varying in 

 number from one to six, and called the pharyngeal bones. 



In the roach, dace, barbel and most other cyprinoid fishes, the 

 teeth are limited to the pharyngeal bones ; in the carp, the upper 

 pharyngeal dental plate is wedged into a cavity of the occipital bone. 

 In the ordinary sharks and rays, on the other hand, the teeth are 

 confined to the maxillary cartilages bounding the anterior aperture 

 of the mouth. The wrasse, (Lahrus), and parrot fish, (Scarus), are 

 instances in which the intermaxillary and premandibular, as well as 

 the pharyngeal bones are provided with teeth, both the anterior and 

 posterior apertures of the mouth being thus surrounded by instru- 

 ments for dividing or comminuting the food. In other fishes, we 

 find the teeth situated not only on the bones which bound the an- 

 terior and posterior orifices of the mouth, but in the intermediate 

 positions, as on the palatines, the vomer, the lingual bones, or the 

 branchial arches ; sometimes, also, but more rarely, on the transverse, 

 or pterygoid, the sphenoid, (1) and the superior maxillary bones, of 

 which latter situation the fishes of the Halecoid(2) tribe and the 

 extinct Lepidotus, afford examples in the present class. Among 

 the anomalous positions of teeth may be cited, in addition to the 

 occipital alveolus of the carp, the accessory rostral cartilages, which 

 in the Pristis are elongated, and so ossified as to be adapted to retain 

 in sockets the strong sharp lateral teeth, constituting its formidable 

 saw. In the lampreys, and in one of the osseous fishes {Helostomus) 

 most of the teeth are attached to the lips. Lastly, I may observe, 

 that it is peculiar to the class of fishes among vertebral animals, to 



(1) PI. 48, fig. 2. (SmcZis). 



(2 In this family M. Agassiz includes the Salinonoid and Clupeoid fishes of Cavier. 



