212 ON THE STRUCTURE OF THE SKULL. 



suprascapular bones, as well as, in some cases, of dorsal dermal 

 bones. 



9. But certain of the most striking modifications of the physi- 

 ognomy of osseous fishes are the result of the prolongation of the 

 region in front of the orbit, which may be effected in two very 

 different ways. For example, it is chiefly the elongation of the 

 preinaxillae and mandible which gives rise to the remarkable 

 beak of the " sword-fish ' (Xiphias) ; while, in Fistularia, the 

 premaxillaB and mandible remain very short, but are thrust out 

 to a great distance from the orbit, by the production of the nasal 

 and vomerine regions, on the one hand, and of the bones of the 

 suspensorium on the other. 



10. In such fishes as Syngnathus and Fistularia, a line join- 

 ing the articular socket of the hvomandibular with the condvle 

 of the os quadratum, makes a very acute angle with the base of 

 the skull. In most fishes this angle is more or less acute ; but 

 in Polypterus, and still more in Muroena, it becomes a right or 

 an obtuse angle, the corner of the gape being thus thrown 

 behind the eye, instead of being, as in most bony fishes, in front 

 of it. We shall find a similar rotation of the distal end of the 

 suspensorium to take place in the series of the Amphibia, and 

 in the passage from the tadpole to the adult state of the 

 highest of these animals. 



11. The connection of the palato-quadrate arcade with the 

 hvomandibular and symplectic suspensor varies, from the firm 

 sutural union which is observed in the Pike and most osseous 

 fishes, to a bond which is hardly closer than that which obtains 

 in the Plagiostomes and Sturgeons, in Polypterus. In Lepidos- 

 teus, except for the inter- and pre-operculum, the tie between 

 the symplectic and the palato-quadrate bones would be very 

 loose, the palato-quadrate arcade and the suspensor being, as it 

 were, naturally dissected from one another. 



In some Pledognathi and Siluroids, on the other hand, all 

 these parts become firmly anchylosed together, and with the side 

 walls of the cranium. 



12. Finally, the multiplication of the bony constituents of 

 the maxilla and the mandible in Lepidosteus — the conversion of 

 the maxilla into a mere support for a tentacle in many Siluroids 



