198 



FISHES 



CHAP. 



substance, but never in the form of true bone. The mode in 

 which the lime is deposited is marked by certain peculiarities 

 which are characteristic of particular families^ (Pig- 113). In 

 some genera, as in the extinct Palaeospinax and the living 

 Acanthias and Scymnus, the calcified portion of each centrum 

 takes the form of a cylinder constricted across the middle, like 

 two cones joined apex to apex (cyclospondylic). This condi- 

 tion is probably the most primitive, but it may be modified in 

 other genera by the further addition of calcic salts in two 

 different ways. Thus, the deposit may take place by the simple 

 addition of concentric layers to the original constricted cylinder 



ha - 



Fig. 113. — Schematic transverse section tiirough tlie middle of a Cyclospondylic (A), a 

 Tectospondylic (B), and an Asterospondylic vertebra (C). d, Middle portion of the 

 calcified double cone ; d\ additional concentric calcified layers ; d'\ double cone 

 "with radiating calcified layers ; ex.m^ external elastic membrane ; h.a, haemal arch ; 

 n.a^ neural arch ; n.c, notochordal cavity. {From Zittel, after Hasse. ) 



(tectospondylic), as in the Slcates and Eays ; or it may take the 

 form of a series of longitudinal plates radiating outwards from 

 the cylinder, and giving rise to a star-like pattern in cross-section 

 (asterospondylic), as in Scyllium and Lamna. In most living 

 Elasmobranchs (e.g. Scyllium), but not in such genera as J^I'oti- 

 danus, Heterodontus, and Squatina, the bases of the dorsal and 

 ventral arches grow round the centra and meet, or even fuse, so that 

 the latter become surrounded by rings of cartilage which, after a 

 fashion, suggest incipient arch-centra (Fig. 112, A). The caudal 

 portion of the vertebral column is often described as "diplo- 

 spondylic," that is, there are two centra, two pairs of basi- 

 dorsals, two pairs of inter-dorsals, and two pairs of basi-ventrals, 



^ Hasse, Das naturliche Syst. d. Elasmolranchier, etc., Jena, 1879, p. 30, et. seq. 



