172 



HOLOCEPHALI 



_-3 



_- 2 



to enclose several occipito-spinal nerves. There is a well-defined 

 convex condyle articulating with the vertebral column. 



The notochord in the Holocephali (Hasse [200], Schauinsland 

 [383], Klaatsch [265]) is persistent and unconstricted. It is 



surrounded by a thick fibrous sheath 

 into which mesoblastic skeletogenous 

 cells migrate through the ruptured 

 elastica externa (Fig. 136). These 

 cells form complete rings, which 

 acquire a calcified bone-like struc- 

 ture, and are much more numerous 

 than the segments of the body. In 

 CaUorhynchus the rings are not de- 

 veloped ; they are, on the contrary, 

 very, strong and closely packed in 

 the extinct Sqiialoraja (Fig. 137). 

 True centra never occur. There 

 are no ribs. Normal cartilaginous 

 neural and haemal arches are present 

 and do not meet round the noto- 

 chord, except in the modified anterior 

 region. Interneurals (interdorsals) 

 and rarely interhaemals (inter- 

 10 ventrals) occur, and supradorsals 

 complete the neural tube above. 

 Immediately behind the skull (Fig. 

 134) some dozen or more vertebral 

 segments are fused below the anterior 

 dorsal fin ; here is formed a con- 

 tinuous cartilage enclosing the noto- 

 chord and nerve-cord, and stretching 

 upwards into a dorsal process with 



whirli arHpnlatfx? tViP p^rfilatrp nf tVip 

 Wm , Cartilage ( 



The axial Cartilage tllbe and 

 o 



*-r 9 



-6 



FIG. 136. 



Chinwcra viwnstrosa, L. Vertebral 

 column: A, transverse section ; B, side 

 view ; and C, longitudinal median section, fin. 



(All after Hasse, slightly modified.) 1, . , , , . , , 



interdorsal ; 2, basjventral ; 3, notochord ; its dorsal prOCCSS may be Considered 



n 

 in 



4, elastica externa and connective tissue ; TO .,,!/ nn nf fne^rl 



5, fibrous sheath ; 6, calcified ring ; 7, basi- as Mafle Up OI lUSCQ 



dorsal ; S, dorsal nerve-root foramen ;<>, living genera its Compound StrUC- 

 vcntral nerve-root foramen ; 10, supra- * 



dorsal. 



. 



ture is shown by the numerous 

 nerve apertures, and in the extinct 



Squaloraja and Myriacanthus traces of segmentation appear in it 

 (Fig. 141, B) (Dean [110ft]). 



The first dorsal fin is characteristically modified, the radii 

 being represented by the single cartilaginous plate just mentioned, 

 to which is firmly fixed the dorsal spine. The whole fin, spine 

 and all, is movable and can be folded back or erected. In 

 Myriacanthus two or three cartilages support the spine. The 



