122 SUB-CLASS ELASMOBRANCHII. 



projects freely on the surface in a backward direction and 

 consists_of__dentine containing a^pulp_aYijty and capped by 

 enamel. The placoicLacales 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 fa^th in T,V>A 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 



t _^~t^ perichondriai ossifi- 



:<jjj/ cation takes place 



X- in some forms in 



the bodies of the 



y J3I vertebrae between 



the arches (see p. 



mSL* 124), but the tissue 



resulting is without 



9H| cells. This does 



', -'.-- '-~^i2 n() t however pre- 



FIG. 66. Longitudinal section through the vertebral column 



of Squatina vulgaris (after Hasse from Gegenbaur). a verte- bone for m -leleos- 



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



lous) ; iv intervertebral ligament ; ch notochord ; fc attenu- , 

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



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

 that the calcined 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. $ 



The vertebraL column (p. 58) presents the most remarkable 



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



t Arch. /. 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 naturliche 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 Wurzburg, 10, 1860, and " Weitere Beobachtungen," etc., 

 Abhdlg. der Senckenberg. Oesellsch. zu Frankjurt, 5. 



