74 



THE ELASMOBRANCH FISHES 



The doubling probably is to be interpreted as meaning that there is need 

 for greater freedom of movement in the active area preceding the caudal fin. 

 In this area where the most severe strain is imposed upon the column it is 

 important, as Ridewood has suggested, that greater strength and at the same 

 time greater freedom of movement be obtained. Strength is given by the in- 

 creased calcification, and freedom of movement is brought about by increasing 

 the nimiber or decreasing the size of the segments; that is, by diplospondyly. 

 Diplospondyly may extend practically to the tip of the tail (Acanthias) or 

 the terminal segments may be irregularly segmented (heterospondylic). 



As a usual thing the vertebrae in the caudal segment of the column present 

 marked regularity in their centra and arches. In Heterodontvs (fig. 78a) the 

 radials are more numerous than are the vertebrae. In Lamna the dorsal 

 radials are inconspicuous, while those having a ventral position are usually 

 large. In this type we see the extreme of heterocercy, in which the axis of the 

 body turns sharply upward into the dorsal lobe of 

 the caudal fin. In Rliinohatis (fig. 78b) the caudal 

 segment is unlike that of the sharks especially in 

 that the vertebral column is here practically straight, 

 and dorsal and ventral radials of the caudal fin skele- 

 ton are of practically equal length. Such a type is 

 more nearly diphycercal than heterocercal. 



Helbing (1904) has shown for Laemargus that 

 there is a tongue of cartilaginous segments in the 

 area between the ventral lobe of the caudal fin and 

 the pelvic fin. This tongue is attached to the haemal 

 process of one of the vertebrae posteriorly and ex- 

 tends anteriorly. While the basal plates of the 

 neural arches are uniform in size, the intercalary 

 pieces are variable. In the anterior part of this area 

 (fig. 78b) they may segment irregularly, while more posteriorly they are con- 

 tinuous with the dorsal radials. At the most posterior tip the intercalaries and 

 radials represent an irregular mass. 



In various regions of the column the calcification takes up difi^erent lo- 

 calities, forming diverse and curious designs. Probably the simplest design 

 is that in which a single ring of calcium is produced in the middle zone of the 

 notochord sheath (fig. 75). A cross-section through a centrum thus calcified 

 shows the ring, of large or small diameter depending upon whether it is cut 

 near the end or at the middle of the centrum. A sagittal section would show it 

 as a broad V above and as an inverted broad V below within the centrum, 

 the two V's being separated at their apices by the notochord. Such a type of 

 vertebra has been designated by Hasse as cydospondijlous (fig. 80, Squalus 

 sucklii). 



Another design formed by the calcification is a further addition to the 

 eyclospondylous type. In this, two or more concentric rings of calcium are 

 formed in the sheath of the notochord. The inner ring is thick like that of the 



Fig. 80. Cyclospondylous 

 vertebra, Sq^ialus sucTclii. 

 ca., calcifieation; chd., 

 notochord; n.c, neural 

 canal. 



