326 CHORDATA. 
The cavity of each auditory capsule opens by a small 
aperture on the dorsal surface. The dorsal wall of the 
cranium is incomplete and the two large openings are known 
as the anterior and posterior fontanelles. 
‘Lhe Visceral Arches form the jaws and the supporting bars 
of the gill region (cf page 417). The principal parts are 
(t) The paired Ayomandibular cartilage, fastened to the 
auditory region of the cranium; (2) The paired padato- 
guadrate cartilage, bound to the distal end of the hyo- 
mandibular. Each has near the hyomandibular a convex 
condyle to which is articulated the mandibular cartilage. 
The two palatoquadrate cartilages form the upper jaw 
and the mandibular cartilages form the lower jaw. Each 
is covered by the placoid scales forming ‘teeth. 
Behind the jaws and attached to the hyomandibular is a 
long jointed Ayord cartilage. Behind this are five branchial 
cartilages on each side, which are joined together ventrally 
by a median plate of cartilage. They support the gills. 
The palatoquadrate and mandibular form the first visceral arch bent 
upon itself, the hyomandibular and hyoid form the second visceral aich 
and the branchials are the third to seventh visceral arches. 
Tn the skate the first two visceral arches, mandibular and hyoid, are 
only loosely attached to the cranium, but in the higher types a shzd/ is 
formed by the fusion of the cranium and these two arches, which latter 
form the facza/ portion of the skull. 
The vertebral column consists of a row of axial cartilages 
from the cranium to the tip of the tail. 
The anterior vertebral plate is a long cartilage which 
articulates anteriorly with the two occipital condyles and pos- 
teriorly with the free vertebre. It has a dorsal (or neural) 
ridge and two lateral ridges, and is pierced by the neural canal 
for transmission of the spinal cord. Behind the vertebral plate 
the vertebral column consists of a series of centra, with a 
hollow facet at each end (amphiccelous). Each has a pair of 
dorsal xewval processes and lateral transverse processes (bear- 
ing small vzbs). These five cartilages are intimately connected 
and lie together below the spinal cord. The neural arch 
over the cord is completed by xeura/ spines lying directly 
dorsal to the centrum and lateral zxterneural plates. In 
the caudal (or tail) portion there are added a pair of hemal 
processes to each centrum and a femal spine. They may 
