900 SUB-KINGDOM VERTEBRATA. 



the united portion forming the ethmonasal septum {ibid., s). By the 

 approximation of the basilar plate to the nasal, orbital, and auditory 

 capsules, these three distinct sense-regions become differentiated in 

 the cranium ; and while the first and third become enclosed in 

 cartilage, the lateral borders of the basilar plate, in some instances, 

 grow upwards to enclose the brain in a complete cartilaginous 

 capsule, which in certain Sharks (fig. 822) persists throughout life. 

 In the higher forms, however, the cartilage does not extend upwards 

 over the brain, which becomes roofed in by bone formed directly 

 from the overlying membrane. 



The visceral portion of the skull is formed by cartilages arranged 

 in a series of arches in the walls of the throat (fig. 820). In the 

 Ichthyopsida there may be as many as nine of these visceral arches, 

 but in the other three classes they become reduced in number to at 

 most three or four, which are also functionally modified. The first 

 arch (fig. 820, m) supports in all cases the walls of the mouth, and 

 is accordingly called the mandibular arch ; the second is termed the 

 hyoid ; while the remaining ones, which persist only in the adults of 

 Fishes as supports of the gills or branchice, are termed branchial. 

 It should further be observed that these arches are separated from 





Orli 



Pf+QK 



Mr7 



Fig. 822.— Left lateral view of the cartilaginous skull or a Shark (Notidanns), greatly reduced. 

 R, Rostrum; AF,PF, Pre- and post-orbital processes; Orb, Orbit; NK, Nasal capsule ; +, 

 Articulation of palatopterygoid (PQ) ; G, Articulation of Meckel's cartilage (Aid) ; Z, Teeth ; 

 WS, Vertebral column. (After Wiedersheim.) 



one another in the embryo by a series of visceral clefts (fig. 820), of 

 which the respiratory apertures of Sharks are remnants. The 7?ian- 

 dibular arch becomes divided into segments, consisting of a short 

 proximal portion known as the quadrate, which very generally forms 

 the main support of the lower jaw ; and a long distal portion known 

 as Meckel's cartilage, around which the mandible, or lower jaw, is sub- 

 sequently formed in those forms which develop membrane bones, but 

 which in the Sharks persists throughout life as the functional lower jaw 



