vill SKULL 207 
erowths 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. ach 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. No 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, Seyllium canicula (Fig. 
120), 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. 
