122 SUB-CLASS ELASMOBRANCHII. 



projects freely on the surface in a backward direction and 

 consists of dentine containing a pulp cavity and capped by 

 enamel. The placoid scales though numerous are not in con- 

 tact, and fresh scales are continually being developed between 

 them, to replace those worn off. The presence of these spines 

 enables the skin of Plagiostomes to be used by polishers 

 (shagreen). The spines are sometimes much enlarged, e.g. the 

 peculiar spines on male skates, the caudal spine of the 

 sting ray {Trygon), the large spines often present on the 

 dorsal fins, etc. The teeth in the mouth are special modifi- 

 cations of placoid scales. 



The endoskeleton is entirely cartilaginous, but the cartilage 



is frequently more or less calcified. 



It is possible that 

 perichondria! ossifi- 

 cation takes place 

 in some forms in 

 the bodies of the 

 vertebrae between 

 the arches (see p. 

 124), bnt the tissue 

 resulting is without 

 cells. This does 

 not however pre- 



„ „ ^ elude its being 



Fia. 66. — Longitudinal section through the vertebral column , t ' T ^ 



of Squat.ina vulgaris (after Hasse from Gegenbaur). a verte- bone, lOr m leieos- 



bral body, showing concentric calcified lamellae (cyclospondy- tei, etc., undoubt- 



lous) ; iv intervertebral ligament ; ch notochord ; fc attenu- rl v. ' 



ated part of notochord. The double calcified cones d are ®^ Done occurs 



shaded dark. without any bone 



cells, or haversian 

 canals. Anatomists are divided on the point, but the preponder- 

 ance of opinion is in favour of the absence of osseous tissue in Elasmo- 

 branchs. KoUiker * takes this view. Gotte f on the other hand holds 

 that the calcified cartilage is true bone. There can be little doubt that 

 bone is quite distinct from cartilage and always arises from elements. out- 

 side it, possibly as dermal plates. These elements may invade the cartilage 

 and bring about so-called cartilaginous ossification or they may always lie 

 outside it, giving rise to membrane bones proper, f 



The vertebral column § (p. 58) presents the most remarkable 



* Ueb. d. Wirbel der Selachier, Abh. Senckenb. Ges. 5. 



t Arch. f. mic. Anat., 1878. 



j Stephan, Bull. Sci. France et Belgique, 1900, p. 281. 



§ For a detailed account of the vertebral column of Plagiostomes "^see 

 C. Hasse, Das natiirliche System der Elasmobranchier, etc., Jena, 1879, 1882 

 and 1885, and especially A. Kolliker " Uber die Beziehung der Chorda zur 

 Bildung der Wirbel der Selachier, " etc., Verhandl. der physik. medic. 

 Oesellsch. zu Wiirzburg, 10, 1860, and " Weitere Beobachtungen," etc., 

 Abhdlg. der Senckenherg. Oesellsch. zu Frankfurt, 5. 



