34 



ANAL AND DORSAL FLNS 



retical dorsal fin, D^ or i)" of Fig. 40. The form of the 

 fin suggests the lobate constriction of the continuous fin 

 web ; its radial supports, Ry 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- 



