THE SKULL OF OSSEOUS FISHES. 109 



the direction towards the median line. It is edentulous in the Cod 

 and most other fishes, but is richly beset with teeth in the Ara- 

 paima gigas. It principally constitutes the floor of the orbit, its 

 breadth depending much upon the depth of that cavity ; it sometimes 

 is joined by its median margin to the vomer and pre-sphenoid, as in 

 the Cod-tribe, Carp-tribe, and Flat-fishes ; and to the basi-sphenoid 

 in Lepidosteus, Erythrinus, and Polypterus, 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 Upencus, Polyprion, and Cheilinis, for example, the ento-pterygoid 

 does not join the basi-sphenoid ; and in Lophiiis it appears to be 

 wanting. 



The pterygoid * {Jig. 30. 24) forms in the Cod an inequilateral 

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

 dovetailed anteriorly ; it becomes thicker towards its posterior end, 

 which is truncated and firmly ingrained with the anterior border of 

 the hypo-tympanic and pre-tympanic bones ; its lower border is smooth, 

 thickened and concave ; edentulous in the Cod, but more frequently 

 supporting teeth, as in the Perch. The pterygoid and palatine appear 

 to form one bone in the great Sudis {Arapaima gigas. Jig. 36. 20, 24) : 

 and they are confluent in the Eel tribe. In the Conger the compound 

 bone is articulated anteriorly to a short lateral process of the vomer, 

 and posteriorly with the hypo-tympanic : it is very large in the 

 Gymnotus. In the Murajna the palatine processes of the vomer 

 do not exist, and the fore part of the long pterygoid is attaclied 

 by ligament to the sides of the vomer, behind its expanded denti- 

 gerous part. 



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

 Osseous Fishes are, in the Cod and most other species, so dispersed, 

 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 dowuAvards and 

 outwards, except those of the last or pterygo-palatine arch, which 

 abut against the tympanic pedicles. The simplification or confluence 

 of the two first of these spurious arches is effected in the Salmonoid 

 Fishes, by the shortening of the premaxillary, and by the mode of its 

 attachment to the maxillary, wliich now forms the larger part of the 

 border of the mouth and supi)orts teeth : the maxillaries are brought 



* Well argued by Dr. Kiistlin not to be, as Ciivier supposed, tlic lioinologuc of 

 the ' OS traiisvL'isum ' of Hcptilcs. — xxxv. pp. 328, 329. 



