VERTEBRAL COLU.MN OF FISHES. 67 



(Plaice, Sole, he, Jig. 20.), and in some parts of the vertebral column 

 of the Deep-fish, as the Dory, the Chaitodon, the Sun-fish, &c. But 

 whatever modifications these dermal and intercalary spines present 

 above, the same are usually repeated below, in connection with the 

 haemal arches and spines, for the support of the anal fin : and just as 

 in the framework of the dorsal fin we find interneural spines and der- 

 moneui'al spines, so in that of the anal fin we recognise interha^mal 

 spines {fig. 19. iJi), and dermoha3mal spines (ib. dh), with the, some- 

 times, expanded base from which they diverge. Both interneural 

 and interh^mal spines are, in the osseous fishes, commonly shaped like 

 little daggers, plunged in the flesh up to the hilt, which is represented 

 by the part to which the true fin-ray (dermoneural or dermohtemal 

 spine) is attached. These parts of the dermal skeleton, developed in 

 the primitive continuous fold of skin which forms the groundwork of 

 the vertical fins in the embryo fish, manifest the vegetative character, 

 which is the usual concomitant of peripheral position, by the partial 

 spontaneous fission which each ray has undergone in the progress of 

 its development ; this is shown by the longitudinal raphe or suture 

 along which each dermal spine or ray may commonly be divided 

 into two lateral moieties. The framework of the caudal fin is 

 composed of the same intercalary and dermal spines, superadded 

 to the proper neural and ha3mal spines, of those caudal vertebrae 

 which have coalesced and been shortened by absorption, in the pro- 

 gress of embryonic development, to form the base of the terminal fin 

 {fig. 18, 19. c, dn, dh). 



In the Sharks and Sturgeons this fin is not symmetrical as in most 

 osseous fishes, but is formed chiefly by the haemal spines and their 

 intercalary and dermal spinous appendages ; the progressively de- 

 creasing bodies of the caudal vertebras are continued along the ujiper 

 border or lobe of the fin, sending off short neural spinous pro- 

 cesses to increase the height of that border. 



M. Agassiz calls those fishes in which, from the peculiar developm(mt 

 of the lower lobe of the caudal fin, the vertebra) seem to be prolonged 

 into the upper lobe, " heterocercal ; " and those with the lobes of the 

 caudal fin equal or symmetrical, he calls " homocei'cal." The pre- 

 ponderance of heterocercal fishes in the seas of the ancient geological 

 epochs of our planet is very remarkable : the prolongation of the 

 superior lobe characterises every fossil fish of the strata anterior to, 

 and including, the Magnesian limestone. The homocercal fishes 

 first appear above that foi'mation, and gradually predominate, until, 

 as in the pi'esent period, the heterocercal bony fishes are almost li- 

 mited to a single ganoid genus, e. g. LepidosteKs. 



The shape, size, and number of the median azygous dorsal and 



F 2 



