EXTERNAL APPEARANCE OF THE SKATE. 417 



The vertebral column consists of an anterior plate not 

 divided into vertebrae, and of a posterior series of distinct 

 vertebral bodies. Each of these consists of a biconcave or 

 amphicoelous centrum. From each side of the centrum a 

 transverse process projects backwards, and bears a minute 

 hint of a rib. From the dorsal surface of each centrum rise 

 two neural processes, which arch upwards on each side of the 

 spinal cord ; the arch is continued upwards in inter-neural 

 plates which meet a neural spine on the top. In the caudal 

 vertebrse, what seem to be the transverse processes are 

 directed downwards, to form a haemal arch enclosing an 

 artery and a vein. In the spaces between the vertebrae lie the 

 gelatinous remains of the notochord. The vertebral column 

 develops from the raesodermic sheath of the notochord. 



The skull is a cartilaginous case, with a spacious cavity 

 for the brain, a large posterior aperture or foramen magnum 

 through which the spinal cord passes, a large ear-capsule on 

 each side posteriorly, a similar nose-capsule on each side 

 anteriorly, a long snout or rostrum projecting in front, two 

 incomplete regions or fontanelles on the roof The skull of 

 the skate develops from (a) a pair of parachordal plates lying 

 on each side of the anterior end of the notochord, {V) from 

 another pair of trabecular plates lying further forward. These 

 coalesce as the floor of the brain-case, and extend upwards 

 and around the brain, being aided (<r) by the ear-capsules 

 and nose-capsules. Furthermore, the skull is connected 

 ventrally with a number of visceral arches. Compared 

 with the skull of a cod or of a higher Vertebrate, that of a 

 skate is simple ; it is not ossified, nor divided into dis- 

 tinct regions, nor has it anything corresponding to the 

 covering or membrane bones, which in higher animals are 

 added to the original foundations of the skull, nor are the 

 visceral arches in the skate directly implicated in forming 

 the skull. 



The arches are primitively supports for the wall of the 

 anterior part of the food-canal, but at least two of them are 

 much modified alike in position and function. 



The upper jaw of the skate is a strong transverse bar, formed 

 from the union of two palato-pterygo-quadrate cartilages. 

 The lower jaw is a similar bar formed from the union of two 

 Meckel's cartilages. 



2 D 



