124 SUB-CLASS ELASMOBRAXCHII. 



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

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

 (In Chlamydosdachus 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. 66). The 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- 



^ c d f e argus borealis the verte- 



brae are entirely uncalci- 

 fied. 



Centra (Fig. 69) in which 

 the internal calcareous mat- 

 c $ ter is confined to the middle 



FIG. 68. Longitudinal section through the hinder zone (double cone) have been 

 part of the vertebral column of Heptanchus, show- pollpH riirln^nnndtilnu^ Tn 

 ing incipient vertebral bodies (e)f and extensive cycLospona^us. 



intervertebral ligaments (a), a ftbro-cartilage some, however, there are 

 of chorda-sheath ; b notocaord ; c calcified double arlrJitirmnl pnnpAntrin pnlpnr- 

 cone; d external calcification; e hyaline cartilage 



of the incipient centrum external to the double eous rings outside the double 

 cone (after Kolliker). cone . such ve rtebral centra 



are called tectospond/ylous. 



In others again there are radiating calcareous lamellae proceeding from 

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

 are termed asterpspondylous. 



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 Lamnidse in which the whole of the 

 chordal sheath is calcified both inside and outside the double 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 frequently 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 (sometimes 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 arches appear to be continued like 



