VIII CAUDAL FIN 237 
dorsal fin of the Devonian Crossopterygian, Holoptychius' (Fig. 
138), where several radiaha, which are free distally, have their 
bases united into a single basal piece, or basipterygium. In 
most Teleostomi elevator and depressor muscles arise from the 
radiaha, and are inserted into different points on the bases of the 
fin-rays, and by their contraction the latter may either be elevated 
into an erect position, or folded back like a fan along the middle 
line of the body, where, as in some Teleosts, there 1s a groove for 
their reception. When fin-rays are only capable of simple eleva- 
tion or depression, the connexion between a radial element and 
its fin-ray is usually by 
some form of a hinge-joint, 
the cleft base of the ray 
clipping the distal segment 
of the radial (Fig. 139). 
In some Teleosts the articu- 
lation of the two is by 
means of a kind of chain- 
link (Fig. 137). In those 
Fishes in which the median 
fins are capable of lateral] Fie. 140.—Caudal end of the vertebral column of 
a Trout (Salmo fario). CN, Centrum; D.F.R, 
undulatory movements the — dermal fin-rays ; H.SP, haemal spine ; H.ZYG, 
articulation is of a more haemal zygapophysis ; N.SP, ene) spine ; 
‘ N.ZYG, neural zygapophysis ; UST, the up- 
mobile character. tilted, partly ossified, and unsegmented ter- 
minal portion of the notochord, or urostyle. 
In the different types (From Parker and Haswell.) 
of caudal fin, diphycercal, 
heterocercal, and homocercal, the supporting elements of the 
ventral lobe are formed by the haemal spines of the terminal 
caudal vertebrae which are inclined backwards, and are often 
ereatly expanded for the purpose (Fig. 140). The dorsal lobe 
may be supported either by the adjacent neural spines, or by 
radiaha, or by both. 
The Appendicular Skeleton.’ It is probable that the skeleton 
of the paired fins and the pectoral and pelvic girdles have been 
formed from the supporting radialia of the isolated and enlarged 
anterior and posterior portions of primitively continuous lateral 
1 Smith Woodward, Brit. Mus. Cat. Foss. Fishes, ii. 1891, p. 335. 
2 W. K. Parker, Shoulder-girdle and Sternum of Vertebrata, Ray Soc. 1868 ; 
Gegenbaur, Untersuch. Vergl. Anat. Wirbelth. Pt. ii. Leipzig, 1865 ; Wiedersheim, 
Das Gliedmassenskelet d. Wirbelth. Jena, 1892. 
