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, 2), 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, Vy, may be seen 
passing to the upper lobe of the tail; the essential out- 
ward form of this truncated, or homocercal, tail had already 
