38 



is a little larger than the right. In the posterior part of each opisthotic bone is a 

 minute opisthotic foramen. In each ex-occipital there is a considerable foramen 

 directed downwards, and behind this a minute foramen directed outwards. 



It now remains to call attention to the form of the outer surface of the skull. The 

 pterotic process projecting outwards from the pterotic bone has already been 

 mentioned. A similar but longer and flatter process projects from the sphenotic 

 bone above the sphenotic foramen ; these processes serve for the attachment of muscles. 

 Below these two processes are two smooth depressions in which are articulated the two 

 heads of the hyomandibular, a bone which is connected with the jaws and branchial 

 apparatus. In the centre of the supra-occipital bone is a somewhat elongated crest. 

 At each side of the dorsal surface of the skuU posteriorly is another crest longitudinally 

 directed, formed partly from the pterotic, partly from the parietal bone : this may be 

 called the parietal crest ; the one on the right is larger than that on the left. These 

 crests also serve for the attachment of muscles. At the anterior part of the basi- 

 occipital bone are a pair of conical prominences with their broad bases downwards : 

 these are continuous with one another in the median plane. The left ectethmoid bone 

 sends off a curved process towards the right side. 



The Vertebral Column. — Any one of the vertebrse from the middle third of the spine 

 consists of a somewhat cylindrical central mass, the centrum, and two elongated 

 processes or spines, a dorsal and a ventral. The centrum has the form of two truncated 

 hollow cones placed with their narrower ends towards each other and united together 

 by a solid disc. The conical hollow at the anterior and posterior surfaces of each 

 centrum with the corresponding hollows of the adjacent vertebrse, enclose cavities 

 which are fiUed by an elastic gelatinous tissue. The lateral depressions at the right 

 and left sides of the centrum are each divided by a longitudinal ridge of bone. The 

 dorsal spine really consists of two hollow bony tubes which diverge at their bases, 

 where they are united with the centrum towards the anterior edge of its dorsal surface, 

 and which are united firmly together in the median plane of the fish's body for the 

 greater part of their length. The diverging legs of the spine form a pointed 

 arch, and through the series of these arches runs the spinal cord, which is thus 

 protected from pressure by the bony arches. Tlie leg of the spine on each side of the 

 vertebra is expanded longitudinally where it joins the centrum, and in this expanded 

 part are two foramina, through which the ventral and dorsal roots of a spinal nerve 

 pass. The ventral spine of the vertebra has a similar structure, but the arch between 

 its legs is larger, and through the series of these arches run blood vessels. There are 

 50 vertcbrse altogether, and the 11th to the 50th have the structure now described. 

 The dorsal and ventral processes in all these are very long, four to five times as long as 

 the centrum. In the first of these vertebrte the spines are almost at right angles to 

 the centrum, .but towards the posterior end they slope more and more backwards. 

 The spines increase in length slightly towards the centre of this region, and again 

 decrease slightly at the posterior end. The centra become longer and narrower 



