THE SKULL OF OSSEOUS FISHES. 101 



medium of a symmetrical cartilage*, which is articulated with the fore 

 part of the nasal boiie, and extends forwards to the interspace of the 

 upper ends of the pre-maxillaries. This 'pre-nasal' cartilage often 

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

 ossified in the Carp. 



In the Mur(Bnid(e the normal elements of the fourth or rhinen- 

 cephalic vertebra coalesce into a single bone: the pre-frontals or 

 neurapophysial elements are plainly manifested, as has been already 

 observed, by the articular surfaces which stand out in front of the 

 orbits for the suspension of the palato-iuaxillary arches : the spine 

 or nasal bone forms the usual obtuse expansion at its anterior extre- 

 mity, immediately beneath the skin of the upper part of the snout, 

 and it supports teeth, as in the Lepidosiren : it is intimately confluent 

 anteriorly with the centrum or vomer, the limits being indicated l)y 

 the interruption of the median series of vomerine and nasal teeth. 



Sense-Capsules. 



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

 are modified to form cavities or orbits 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 vertebrae. 



Acoustic capsule, or Petrosal^ {J^g- 30. IG). 



"We have seen that the first developed cartilage upon the primitive 

 membranous walls of the skull forms a special protecting envelope 

 for the labyrinth, which alone constitutes the organ of hearing in 

 Fishes (Ammocetes, Jiff. 24. 16). In the progressive accumulation of 

 cartilaginous tissue upon the base and sides of the cranium, the ear- 

 capsule loses its individuality, and becomes buried in the common 

 thick basi-lateral parietes of the cranium. It is blended with that 

 persistent cartilaginous part of the skull in the Lepidosiren ; but, in 

 the better ossified Fishes, when the osseous centres of the neura- 

 pophyses of the cranial vertebra) begin to be established in that car- 

 tilaginous basis, a distinct bone is likewise, in most cases, develoi^ed 

 for the more express defence of the labyrinth. Since, however, 



* This is regarded by some honiologists as tlic bodj' of a fifth cranial vertebra; 

 but from its relations to the nasal l)one it would have better claims to be considered 

 the spine of such, if tliere were suflicient grounds for admitting vertebral segments 

 beyond the nasal one: and the cephalic region of the skeleton miglit well diller, 

 like the cervical and other regions, in the number of its vertebral segments; but I 

 have not found good evidence of such variation. 



•j- Iioili(r, Cnvier; riipciil, CJcollVoy ; pars pclrosa nssh- /cw/ior/*- of Anthropotomy. 

 The nature of tiio ' os petrosiim ' as an envelo])e of the acoustic hulb, and its 

 serial homology with (he sclerotic capsule of the optic Imlb, are clearly enunciated 

 l)y Professor lie I51ainville, in the first |)art of his great " Osteographic," I to. lf^39, 

 pp. 1:5. -J^. 



U 3 



