COMPOSITION OF THE SKULL. 



379 



ing gill-slits or spiracles, but are closed in the higher Verte- 

 brates. As a rule, the skull is symmetrical, exceptions being 

 found in the flounders and the bones about the nose of cer- 



Pig 381. -Skull of the Lion. 2, occipital condyle ; 7, Parietal bone and sagittal 

 crest ; 8, paroccipital ; 27', squaraosal bone ; 27, zygomatic arch ; 26, malar bone ; 

 11, frontal bone ; 12, post-orbital process; 15. nasal bone; 21, maxillary bone; 22, 

 premaxillary bone ; 32, mandible ; 3, occipital crest ; c, canine teeth ; /'-', second pre- 

 molar ; nil, molar tooth. After Owen. 



tain whales and porpoises. The base of the skull is perfo- 

 rated for the exit of the nerves proceeding from the base of 

 the brain, and the hinder bone (occiput] is perforated (fora- 

 men magnum) for 

 the passage of the 

 spinal cord from the 

 medulla oblongata. 

 It is probable that 

 there is a general 

 parallelism between 

 the head of Insects 

 a n d Vertebrates. 



While the head of Fi & 382. Skull and vi-ceral skeleton of a Selachian 



, . (diagram), on: occipital region; la, wall of the laby- 



Wlllged insects, for rintl\ ; >//>. ethmoidai region ; n, nasal pit ; a, first, b, c, 



,, second labial cartilage ; o, superior, n, inferior portion 



example, Consists OI ot the maudibulararch/. ; //., hyoid arch; III.-V11I. 



u (1-6), branchial arches. After Gegenbaur. 

 a certain number or 



segments, homologous with those of the rest of the body, 

 and with mouth-parts homologous with the limbs ; so the 

 skull is also segmented, and an expansion and continuation 

 of the vertebral column. Gegenbaur even maintains that 

 the various arches of the head are homologous with the limbs. 



01 



VJ11 



