120 ANATOMY OF VERTEBRATES. 



other fishes, but is richly beset with teeth in the Arapaima gigas. 

 It principally constitutes the floor of the orbit, its breadth de- 

 pending much upon the depth of that cavity ; it sometimes is 

 joined by its median margin to the vomer and presphenoid, as in 

 the Cod-tribe, Carp-tribe, and Flat-fishes ; and to the basisphenoid 

 in Lepidosteus, Erythrinus, and Pofypterus, and then divides the 

 orbit from the mouth ; but more commonly a vacuity here exists 

 in the bony skull, filled up only by mucous membrane in the 

 recent fish ; in Upeneus, Polyprion, and Cheilinis, for example, 

 the entopterygoid does not join the basisphenoid. 



The pterygoid forms in the Cod, fig. 75, 24, an inequilateral 

 triangular plate, but more elongated than the palatine, with which 

 it is dovetailed anteriorly ; it becomes thicker towards its pos- 

 terior end, which is truncated and firmly ingrained with the 

 anterior border of the hypotympanic ; its lower border is smooth, 

 thickened, and concave ; edentulous in the Cod, but more fre- 

 quently supporting teeth, as in the Perch. The pterygoid and 

 palatine appear to form one bone in the great Sudis, (Arapaima 

 gigas, fig. 86, 20, 24): and they are confluent in the Eel tribe. 



The ten bones of which the palato-maxillary arch is composed 

 in Osseous fishes are, in the Cod and most other species, so dis- 

 posed, in relation to the peculiar movements of the mouth, as to 

 appear like three parallel and independent arches, successively 

 attached behind one another, by their keystones, to the fore part 

 of the axis of the skull, and with their piers or crura suspended 

 freely downward and outward, fig. 75, 22, 21, except those of the 

 last or pterygo-palatine arch, ib. 23, 24, which abut against the 

 tympanic pedicles. The simplification or confluence of the two 

 first of these spurious arches is eifected in the Salmonoid Fishes, 

 Sudis, fig. 86, &c., by the shortening of the premaxillary, and by 

 the mode of its attachment to the maxillary, which now forms 

 the larger part of the border of the mouth and supports teeth : 

 the maxillaries are brought into close articulation with the pala- 

 tines in the Plectoo-nathes, and the consolidation of the whole 



O ' 



series into its normal unity is effected in the Lepidosiren. The 

 palatines form the true bases of the inverted arch at their points 

 of attachment to the prefrontals ; the premaxillaries constitute the 

 true apex, at their mutual junction or symphysis ; the approxi- 

 mation of which to the anterior end of the axis of the skull is 

 rendered possible in fishes, by the absence of any air-passage or 

 nasal canal ; the pterygoids are the diverging appendages of the 

 arch ; but are attached posteriorly to strengthen the pedicle sup- 

 porting the lower jaw, and combine its movements with those of 



