36 



CAUDAL FIN 



dorsal and ventral sides, but, unlike the radials of the anal 

 or dorsal fins, do not segment off basal elements. They 

 first occur in the region of the base of the caudal, as in 

 the embryonic stage (Fig. 44, R), since, perhaps, it is in this 

 region that the greatest stress occurs in propulsion. It 

 is not until a later stage that their metameral sequence 

 is extended backward to the tip of the vertebral axis 

 (Fig. 40, C). 



With the origin of cartilaginous supports there seems 

 to have arisen a mechanical need for enlarging the ventral 

 lobe of the caudal ; it is here certainly that in the majority 

 of early forms the radials appear longer and stouter, giv- 

 ing rise to the condition of heterocercy of Figs. 45 and 46. 

 The greater functional importance of the radials of the 

 ventral region, R + H, is acquired contemporaneously with 

 the upturning of the end of the vertebral axis. In the 

 tail of a Lower Carboniferous shark (Fig. 46, v. p. 79), an 

 extreme degree of heterocercy has been acquired before 

 the radials of the lower lobe have extended themselves in 

 the hindmost region of the vertebral axis ; the ventral web 

 of the upper tail lobe, accordingly, is still strengthened 

 by minute (dermal) rays, which the writer believes homol- 

 ogous with actinotrichia ; on the fin's dorsal side the 

 radials have been abruptly upturned with the notochord, 

 and are fused into a compact cutwater. 



The plan of structure of the shark's caudal fin (Fig. 45) 

 may in its most primitive form prove to be the ancestral 

 one of fishes ; if this is the case it would give rise to the 

 types of caudal fins of Figs. 47 and 48. That it has given 

 rise to the latter form cannot be doubted, for even in the 

 adult condition of the fin the notochord, N, may be seen 

 passing to the upper lobe of the tail ; the essential out- 

 ward form of this truncated, or homocercal, tail had already 



