BY WILLIAM A. HASWELL, M.A., B.SC. 75 



rorwards the line of the dorsal spines ; on each side is another 

 ridge apparently continuous with the transverse processes, and 

 a row of small foramina (through which pass divisions of the 

 vagus nerve), which seem to continue forwards the row of 

 spinal nerve-foramina. In a few other sharks ( Spinacidcu) these 

 peculiarities are traceable in a less decided form ; but in the 

 majority the mesial and lateral occipital ric'ges disappear. In 

 front of the occipital is the auditory region of the skull, in which 

 are contained the various divisions of the membranous labyrinth, 

 and with which articulate laterally the hyo-mandibular cartilages. 

 In the lower forms among the Plagiostomes the surface of the 

 auditory region takes its form to some extent from that of the 

 enclosed parts of the auditory apparatus, and elevations marking 

 the position of the anterior and posterior semi-circular canals and 

 of the vestibule are very prominent on the surface ; in higher 

 forms these elevations become less marked, and in some exceptional 

 instances they may become so obscured as to be barely traceable. 

 On the upper surface of this segment of the skull are the two small 

 apertures of the aqueductus vestibuli ; in the Sharks these two 

 apertures are closely approximated to one another, and are situated 

 at the bottom of a common pit or groove — the parietal groove. 

 In the Rays, on the other hand, the two apertures are separated 

 from one another by a distinct interval and do not lie in any 

 common groove. 



The articular surface for the hyo-mandibular in the lateral wall 

 of the auditory region varies greatly in its form and position. In 

 form it varies from a simple concavity to a complex articular 

 surface, sometimes divided into two parts ; it may be excavated on 

 the lateral wall of the auditory region, or may be borne out from 

 the general surface on prominent processes of the postero-lateral 

 region of the cranium. In the Rays it is placed nearer the, base of 

 the cranium than in the Sharks, and is more elongated antero- 

 posteriorly. Behind it in the Rays is sometimes a small articular 

 surface for the first branchial arch. 



In front of the auditory region is the orbital region, which 

 presents on either side a deep concavity for the reception of the eye. 



