viii SKULL 2O7 



growths from the mesethmoid plate. Later, the parachordals and 

 trabeculae grow upwards on each side round the brain, and to a 

 greater or less extent they meet and fuse on its dorsal surface, 

 thus enclosing the latter organ in a cranial cavity, leaving, 

 nevertheless, a large foramen behind (foramen magnum) through 

 which the brain is continuous with the spinal cord. In this 

 condition the primitive cartilaginous cranium, with its included 

 sense-capsules, has reached a stage which is permanently retained 

 in such Fishes as the Elasmobranchs. 



The visceral arches consist of a number of pairs of curved 

 rods of cartilage, at first simple, but subsequently segmented, and 

 developed in the splanchnic mesoblastic walls of the oral cavity 

 and pharynx. Each rod is connected with its fellow by a 

 median cartilage in the floor of the pharynx, so that the whole 

 form a series of dorsally incomplete hoops encircling the anterior 

 portion of the alimentary canal. Xo doubt all the visceral 

 arches were originally branchial arches, and were so disposed 

 between the successive gill-clefts as to support their walls and 

 the vascular folds or gill-lamellae to which they gave rise. In 

 Fishes most of the arches still retain their primitive gill- 

 supporting function, but the first or mandibular arch has become 

 modified to form upper and lower jaws, although in the Sharks 

 and Dog-Fishes it may lie in front of a "gill-cleft and still be 

 associated with vestigial gills. The second or hyoid arch is less 

 removed from the condition of a branchial arch, and generally 

 supports either a functional or a vestigial gill, but in most 

 Fishes it has acquired the secondary function of forming a 

 suspensorium for the attachment of the jaws to the cranium. 



The skull of the common Dog-Fish, Scyllium canicula (Fig. 

 120), 1 may be studied as a type which in the adult remains carti- 

 laginous, and has no secondary addition of cartilage- or membrane- 

 bones. In this Fish the chondrocranium, or primary cartilaginous 

 cranium, presents the appearance of a somewhat depressed oblong 

 box, which has a complete roof, side-walls, and floor, but is open 

 in front (anterior cranial fontanelle) and also behind (foramen 

 magnum}. The hinder, or parachordal portion of the cranium 

 surrounds the foramen magnum, and there forms the occipital 

 region. At the ventral margin of the foramen there are two 

 prominences, or occipital condyles, for articulation with the first 



1 W. K. Parker, Trans. Zool. Soc. x. 1878, p. 189. 



