114 Al^ATOMY OF VERTEBRATES. 



basicranial canal. In some fishes, as the SalmonidcB, the nasal is 

 broad but not deep : in Istiophorus it is long and narrow : in the 

 Discoboles and Loplwhranchii it is a short vertical compressed 

 plate : it is altogether absent in the Lophius, or is represented 

 here, as in the Diodon, by a fibrous membrane, retaining the 

 primitive liistological condition of the skeleton. In the Flying 

 Gurnard the nasal has no immediate connection with the vomer ; 

 but this is a rare exception. In most fishes the nasal canity is 

 more completely divided by the nasal bone into two distinct lateral 

 fossa3 than in any other class of Vertebrates. 



In Amia, Lepidosteus, Polypterus, and many extinct ganoid 

 Fishes the nasal is divided at the median line. The horn-like pro- 

 jection from the fore part of the skull of the Naseus unicornis is 

 formed chiefly by a process of the frontal bone, to the under part 

 of which a small nasal is articulated. 



The turbinals, or osseous capsules of the nose, are situated at the 

 sides or above the nasal: the premaxillary and the maxillary 

 bones are usually attached to its extremity through the medium of 

 a symmetrical cartilage which is articulated with the fore part of 

 the nasal bone, and extends forward to the interspace of the upper 

 ends of the premaxillaries. This ' prenasal ' cartilage often forms 

 a septum between the two ' ossa turbinata : ' it is partially ossified 

 in the Carp. 



The sense-capsules are so intercalated with the neural arches, 

 which are modified to form cavities for their reception, that the 

 demonstration of the skull will be best facilitated by describing 

 them before we proceed to the hajmal arches of the cranial verte- 

 brte. 



Acoustic capsule, or petrosal, figs. 81, 83, 85, 16. 



We have seen that the first developed cartilage upon the 

 primitive membranous wall of the skull forms a special protecting 

 envelope for the labyrinth, which alone constitutes the organ of 

 hearing in Fishes (Ammocetes, fig. 58, le). In the progressive 

 accumulation of cartilaginous tissue upon the base and sides of 

 the cranium, the ear-capsule loses its individuality, and becomes 

 bu^ried in the common thick basilateral parietes of the cranium. 

 It is blended with that persistent cartilaginous part of the skull in 

 the Lepidosircn ; but, in the better ossified Fishes, when the 

 osseous centres of tlie neurapophyscs of the cranial vertebra; 

 begin to be establislied in that cartilaginous basis, a distinct bone 

 is likewise, in most cases, developed for the more express defence 

 of the labyrinth. Since, however, fiuictious are less specialised, 

 less confined to the particular organ ultimately destined for their 



