34 ANAL AND DORSAL FINS 
retical dorsal fin, D' or D" of Fig. 40. The form of the 
fin suggests the lobate constriction of the continuous fin 
web ; its radial supports, #, extend from the body wall to 
the margin of the fin, and between them traces of actino- 
trichia are to be seen. The anterior margin of the fin 
must now function as a strong cutwater, its supporting 
elements, both radial and basal, tightly clustering. A fin 
of this character could evidently have possessed a greater 
freedom of lateral movement in its hinder than in its an- 
terior part; and thus the clustering of the fin supports 
becomes of especial significance. The region of move- 
ment, restricting itself to the hinder part of the fin, 
permits extensive fusions of the supporting cartilages 
anteriorly, and leads ultimately to exceedingly complex 
conditions. The dorsal fin of a Coal Measures fish (Ho- 
loptychius, p. 151) has thus (Fig. 43) specialized the power 
of lateral movement in the highest degree. The length 
of the fin has, in the first place, become greatly compressed, 
a process which seems to have resulted in implanting the 
anterior basals, B, deeply into the integument and in 
fusing them: the posterior basals then appear to have 
been everted from the surface of the body. Here they 
still retain their segmental arrangement, but are irregular — 
in shape and reduce in size distally. : 
An important part is taken by the dermal margin of 
the fin in modifying the size of the older fin supports. 
The simplest form of a dorsal fin of a recent shark (Fig. 
42) has thus more than half of its functional area of a 
dermal origin, although, in other regards it resembles — ¢ 
closely the conditions of Fig. 41. The dermal margin of 
the fin has apparently increased to the detriment and 
consequent reduction of the cartilaginous elements; it 
produces in its secondary structures light flexible horn- 
