234 FISHES CHAP. 
Median Fins and Appendicular Skeleton 
The Median Fins.—Whether existing in the form of a 
continuous fin, or as discontinuous isolated fins, the median 
fins are provided with skeletal supports, and also with muscles, 
primitively formed from intrusive clusters of cells derived 
from a variable number of the neighbouring myotomes, for 
their varied movements. The skeletal structures of the dorsal 
and anal fins consist of a series of bony or cartilaginous, rod- 
like, and typically tri-segmented radial elements or pterygio- 
phores,’ supporting distally a series of dermal structures in 
the shape of numerous slender horny fibres or ceratotrichia, 
as in the Elasmobranchii and Holocephali, or a smaller number 
of bony dermal fin- 
rays, which are prob- 
ably modified scales 
or lepidotrichia,’ as in 
the Teleostomi. The 
typical tri-segmented 
character of the radi- 
alia is often retained 
in many existing Elas- 
mobranchs (Fig. 155) 
and in Pleuracan- 
thus, in Neoceratodus 
amongst the Dipnoi, 
in the Chondrostei, 
in existing Holostei 
Fic. 135.—The cartilaginous radialia of the first dorsal oe 2 , 
fin of Mustelus antarcticus. (From Mivart.) (F 1g- 13 6), and to a 
greater or less extent 
in several families of Teleosts (eg. Salmonidae, Esocidae, 
Cyprinidae, and some Acanthopterygii); but in the latter 
group the radialia are greatly prone to reduction, and hence 
they are more generally bi-segmented, and sometimes consist 
of a single proximal segment only (eg. Gymnotus). In 
all these Fishes the proximal segments are the longest 
and the most persistent, and when reduction occurs it is at 
' Thacker, Zrans. Connecticut Acad. iii. 1877, p. 281; Mivart, Trans. Zool. 
Soc. x. 1879, p. 439; Bridge, Linn. Soc. Journ. Zool. xxv. 1896, p. 530. 
* Goodrich, Quart. Jowrn. Micr. Sci. 47, 1903-1904, p. 465. 
