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- 
SAMS 
52 
» 
re 881.—Skull of the Lion. 2, occipital condyle ; 7, Parietal bone and sagittal 
.crest ; 8, paroccipital ; 27, squamosal 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 ; p®, second pre- 
molar ; ml, 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 
and Vertebrates. 
While the head of Fig. 38%.—Skuli and visceral skeleton of a Selachian 
é * (diagram). occ, occipital region; da, wall of the laby- 
winged insects, for rint j llethnoidal region ; 7, nasal pit ; a, first, b, ¢, 
* second labial cartilage ; 0, superior, 2, inferior portion 
example, consists of of the mandibular arch f,; Z/., hyoid arch; ZZJ.- VII. 
a certain number of (1-6), branchial arches.—After Gegenbaur. 
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, 
