336 MANUAL OF ZOOLOGY SECT. 



region of the cranium articulates, usually, movably, with the 

 first vertebra of the spinal column. In addition to the 

 cranium the skull comprises certain elements known as the 

 visceral arches. The foremost of these forms the jaws, the 

 second is the hyoid, and mainly supports the tongue, the 

 remainder are the branchial arches. 



In the Dog-fish (Fig. 198) the cranium remains in the 

 primitive condition of a cartilaginous case, with complete 

 walls and floor, but with the roof partly formed of fibrous 

 membrane. In the Lizard and Rabbit the substance of the 

 cartilage is replaced by a number of cartilage bones, i.e., 

 bones which take the place of pre-existing cartilage, to which 

 are superadded a number of membrane bones, i.e., bones, the 

 site of which was not pre-occupied by cartilage ; the whole 

 united together so as to form a structure of considerable 

 complexity. The visceral arches in the Dog-fish are com- 

 posed of a system of rods of cartilage. The first visceral 

 arch forms the upper and lower jaws (up. j., /./.), between 

 which the opening of the mouth is situated. The jaws are 

 connected on each side with the skull behind by means of a 

 cartilage known as the hyomandibular (hy. ;;/.), which is a 

 part of the second or hyoid arch ; the rest of the hyoid 

 arch and the branchial arches, which are five in number, lie 

 in the lateral and ventral walls of the pharynx and support 

 the gills. 



In both the Lizard and the Rabbit the branchial arches 

 are not present as such, the only well-developed visceral 

 arches being the first and second. The upper jaw is 

 formed of certain membrane bones, and in the lower jaw 

 also the cartilage completely disappears, its place being 

 taken by bones which are early united together, so as to 

 form the bony lower jaw or mandible. In the Lizard (Fig. 

 199) the mandible articulates on each side with the posterior 



