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’ (Fig. 113). In 
some genera, as in the extinct Palaeospinaz and the living 
Acanthias and Seymnus, 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 eylinder 
Fic. 113.—Schematic transverse section through the middle of a Cyclospondylic (A), a 
Tectospondylie (B), and an Asterospondylic vertebra (C). ad, Middle portion of the 
calcified double cone ; d’, additional concentric calcified layers ; d’, double cone 
with radiating calcified layers ; ex.m, external elastic membrane ; /.a, haemal arch ; 
n.d, neural arch ; 7.c, notochordal cavity. (From Zittel, after Hasse. ) 
(tectospondylic), as in the Skates and Rays; 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 Seyllium and Lamna. In most living 
Elasmobranchs (e.g. Seyllcwm), but not in such genera as Nott- 
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- 
spondyhec,” that is, there are two centra, two pairs of basi- 
dorsals, two pairs of inter-dorsals, and two pairs of basi-ventrals, 
1 Hasse, Das natiirliche Syst. d. Elasmobranchier, ete., Jena, 1879, p. 30, et. seg. 
