124 



SUB-CLASS ELASMOBRAXCHII. 



of tlie vertebral regions (as shown by the arches) it has thickened, 

 become firmer, and has encroached upon the notochord (Fig. 68). 

 (In Chlamydoselachus this only happens in front, the notochord 

 is unconstricted behind, and in Heptanchus the chordal sheath 

 has segmented in the caudal region.) 



In other Plagiostomes the notochordal sheath is segmented 

 into alternately cartilaginous portions which constitute the bodies 

 of the vertebrae and fibrous parts which are the intervertebral 

 ligaments (Fig. 60). Tlie sheath thickens in the centre of the 

 vertebral portions and constricts the notochord ; moreover its 

 tissue consists of three layers, forming the inner, middle and 

 outer zones. The middle zone is fibrous and is nearly always 

 calcified, forming the so-called double calcareous cone (Fig. 66, c). 



In Hexanchus and Laem- 

 argus borealis the verte- 

 brae are entirely uncalci- 

 fied. 



VlQ. 68. — Longitudinal section tlirough tlie hinder 

 part of the vertebral column of Heptanchus, show- 

 ing incipient vertebral bodies (e), and extensive 

 intervertebral I'gaments (a), a flbro-cartilage 

 of chorda-sheath ; 6 notocuord ; c calcified double 

 cone ; d external calcification ; e hyaline cartilage 

 of the incipient centrum external to the double 

 cone (after Kolliker). 



Centra (Fig. 69) in which 

 the internal calcareous mat- 

 ter is confined to the middle 

 zone (double cone) have been 

 called cyclospondylous. In 

 some, however, there are 

 additional concentric calcar- 

 eous rings outside the double 

 cone ; such vertebral centra 

 are called tectospondylous. 

 In others again there are radiating calcareous lamellae proceeding from 

 the double cone through the outer zone of the chordal cartilage. Such 

 are termed asterospondylous. 



The amount of calcification varies from the cyclospondylous condition 

 of the simple double cone with soft tissue outside and inside, to the condi- 

 tion found in many Raji and in the Lamnida3 in which the whole of the 

 chordal sheath is calcified both inside and outside the dovible cone. The 

 first calcification is always in the chordal sheath in the fibrous tissue of the 

 middle zone ^double cone). Later arise the concentric rings of the Tecto- 

 spondyli or the radiating lamellae of the Asterospondyli ; these are calcifica- 

 tions of the hyaline cartilage of the chordal sheath. When skeletogenous 

 tissue is developed outside the elastica externa either by an extension 

 of the cartilage of the arches over the centra (many Cyclospondyli), or 

 {Carchariidae, etc.) by development on the inside of the perichondrium of 

 the centrum, it freqviently becomes calcified as four wedges, which extend a 

 certain distance into the body between the insertion of the arches. These 

 hard wedges may or may not reach the double cone (sometinies they do 

 so at the end and not at the centre of a vertebral body) ; they give rise to 

 the cruciform radiating figure seen in the section of some vertebrae. In 

 such cases the uncalcified cartilaginous arclies appear to be continvied like 



