326 CHORD AT A. 



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. 



'I'he Visceral Arches form the jaws and the supporting bars 

 of the gill region (cf. page 417). The principal parts are 

 (i) The paired hyomandibular cartilage, fastened to the 

 auditory region of the cranium ; (2) The paired palato- 

 quadrate 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 hyoid cartilage. Behind this are fv\Q 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. 



In the skate the first two visceral arches, mandibular and hyoid, are 

 only loosely attached to the cranium, but in the higher types a skull is 

 formed by the fusion of the cranium and these two arches, which latter 

 form XhQ facial 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 vertebrae. 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 (amphicoelous). Each has a pair of 

 dorsal neural processes and lateral transverse pjvcesses (bear- 

 ing small rit?s). These five cartilages are intimately connected 

 and lie together below the spinal cord. The neural arch 

 over the cord is completed by neural spines lying directly 

 dorsal to the centrum and lateral interneural plates. In 

 the caudal (or tail) portion there are added a pair of hcEmal 

 processes to each centrum and a hcemal spine. They may 



