CAUDAL FIN 



237 



dorsal fin of the Devonian Crospopterygian, -Holoptycliius ^ (Fig. 

 138), where several radialia, 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 

 radialia, 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 is 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 Fig. UO.-Caudal end of the yertehral column of 

 ^ ^ , J ji A TtovX (Salmo fario). CN, Centrum ; U.t .li, 



dermal fin-rays ; H.SP, haemal spine ; H.ZYG, 

 haemal zygapophysis ; N.SP, neural spine ; 

 N.ZYG, neural zygapophysis ; UST, the up- 

 tilted, partly ossified, and unsegmented ter- 

 minal portion of the iiotochord, or urostyle. 

 (From Parlcer and Haswell.) 



undulatory movements the 

 articulation is of a more 

 mobile character. 



In the different types 

 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 

 greatly expanded for the purpose (Fig. 140). The dorsal lobe 

 may be supported either by the adjacent neural spines, or by 

 radialia, 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 



> Smith Woodward, Brit. Mus. Cat. Foss. Fishes, ii. 1891, p. 335. 



' W. K. Parker, Shoulder-girdle and Sternum of Vertebrata, Ray See. 1868 ; 

 Gegenbaur,. Untersuch. Vergl. Anat. Wirbelth. Pt. ii. Lei^jzig, 1865 ; Wiederaheim, 

 Das GUedviassensTcelet d. Wirbelth. Jena, 1892. 



