THE SKATE. 29 



ossifications are deposited in the form of minute polygonal 

 plates, forming a mosaic over the whole surface. 



III. In the vertebral column, make out 



3. .The anterior part of the column, consisting of a large 

 mass of cartilage — the anterior vertebral plate (Fig. 10, 

 a.v.p), — not divided into distinct vertebrae, except in its 

 posterior ventral portion, where there are rudimentary 

 centra. It presents an almost flat ventral surface, a strong 

 median dorsal ridge, and two lateral ridges, which, at about 

 the middle of the plate, are produced upwards, so as to be 

 nearly as high as the median ridge. The neural canal, for 

 the reception of the spinal cord, runs from end to end of the 

 plate, the side walls of which are perforated by two rows of 

 small apertures, for the passage of the dorsal and ventral 

 roots respectively of the spinal nerves. The antero-inferior 

 region of the vertebral plate is produced into a scoop-like 

 odontoid process, which fits between the condyles of the 

 skull (§ 19): on either side of this, on the front boundary 

 of the lateral ridge, is a smooth surface, with which one of 

 the condyles themselves articulates. 



4. The remainder of the vertebral column, consisting of 

 distinct vertebrae, which are divided into two sets : the 

 trunk vertebrae, extending from the posterior end of the 

 vertebral plate to the root of the tail, and the caudal 

 vertebrae, reaching to the end of that appendage. 



IV. Examine the trunk vertebrae ; some from the sur- 

 face ; some by means of longitudinal and trans- 

 verse sections ; some by disarticulation from the 

 rest ; and others by allowing to dry. Make out 



5. The centra of the vertebras (Figs, n, 12, and 13, c), 

 cylindrical bodies with deeply-concave anterior and pos- 

 terior ends ; these ends are formed by concavo-convex 

 plates of bone, which meet one another in the middle of the 



