SKULL OF VERTEBEATA. 449 



arches- are embedded in tlie vipper and lower lip, in front of the 

 mandibular arch ; they are the labial cartilages. 



The parts, therefore, -which are derived from the branchial 

 skeleton and enter into closer relation with the skull are these : 



1) The two labial cartilages (Fig. 240, a and h c) ; the anterior 

 one consists of one, the posterior of two pieces. 



2) The mandibular arch (F), which consists of a superior piece — 

 palato-quadrate (o), and of an inferior piece — lower jaw (?t). 



3) The hyoid arch (IT) ; of this the upper portion only (hyo- 

 mandibular) has any intimate relations to the skull. 



We find various cartilaginous rods attached to all the arches, 

 except the labial cartilages ; these are directed backwards, and afford 

 support for the branchial pouches; they are known as branchial 

 rays. They undergo various modifications; they are diminished 

 in number on the palato-quadrate, but may be seen in the wall 

 of the spiracular cleft, which represents a rudimentary branchial 

 pouch (spiracular cartilage). 



We shall treat of these portions of the branchial skeleton with 

 the skull ; the other arches {III — YIII) will be considered later on 

 (§ 353). 



The arrangement we have described may be seen in the cephalic 

 skeleton of the Selachii. All its parts are formed of cartilage, which 

 is, as a rule, covered over by a thin calcified layer, which however is 

 never ossified. In the cartilaginous cranial capsule the most anterior 

 portion is formed by the ethmoidal region. On its inferior surface 

 there is a nasal pit (n) on either side. The cranial cartilage is 

 frequently continued forwards, and between these pits, into a process 

 (I'ostrum). The succeeding portion, with its cavities, forms the 

 orbits, which is overhung by cartilage above and behind. The next 

 portion (Za) is the broadest ; at the sides this encloses the auditory 

 labyrinth, and is continuous at its hinder surface with the last of the 

 primary regions {occ) ; in some Sharks (Notidani) this hindermost 

 region is continuous with the vertebral column. 



Teeth are developed in the mucous membrane which covers the 

 palato-quadrate, as well as in that which covers the lower jaw; 

 and this explains why these cartilages are so well developed. 

 The hyomandibular is attached posteriorly to the palato-quadrate, 

 and is either directly continuous with the lower part of the hyoid 

 arch (Notidani), or is more freely attached to it. The hyomandibular 

 is consequently greatly developed in the Sharks, and is converted 

 into a kind of suspensorium, by gradually gaining a closer connec- 

 tion with the lower jaw. Finally the hyoid portion ceases to appear 

 to be a continuation of the hyomandibular ; in the Rays, indeed, it 

 loses all connection with it, and the hyomandibular is then merely 

 the support of the jaws. 



The skull of the Chimserae is very different to this, for in it 

 there is a continuous connection between the jDalato-quadrate portion 

 of the first arch and the cartilaginous cranium. The mandibular 

 portion, which is articulated with a process of the cranium, is the 



2 G 



