49 
8.—CaupaL Fin anp EXTREMITY or VERTEBRAL CoLUMN 
(Figs. 14, 15 and 19). 
The caudal fin when first dissected appears to be 
diphycercal, but the asymmetrical articulation of the 
second hypural bone (fH .2) at once establishes its 
heterocercal character. Nevertheless the caudal fin of the 
adult Plaice is one of the most completely masked hetero- 
cercal or homocercal fins on record, and is hence of some 
interest. The termination of the vertebral column is 
peculiar, as in place of an upturned tapering bony 
“urostyle ” formed by the ossification of the free extremity 
of the notochord, there is found an expanded fan-shaped 
plate which, with the second hypural, gives articulation to 
the greater part of the caudal fin rays. The proximal 
stout vertebra-like body articulating with the last true 
vertebra may possibly represent another vertebra, but it is 
not constricted off in Plaice from 15 mm. upwards, and in 
the absence of earlier developmental evidence is here 
described as the base of the urostyle. Dissection of young 
plaice of a length of 45mm. shows the vertebral column 
terminating as above described, but similar preparations 
of still smaller forms of 15-17mm., where the notochord is 
yet unossified, prove conclusively that the fan-shaped plate 
is formed by the fusion of the upturned extremity of the 
notochord with a separate hypural bone (fig. 15, Hp.3), 
and hence the plate (U. + Hp.3) of the adult is a com- 
pound bone representing the urostyle and a third hypural 
fused together. It will be noticed in fig. 15 that the free 
surface for articulation with the fin rays is afforded exclu- 
sively by the third hypural—the urostyle taking no part 
in it. Apart from the independence of the urostyle and 
third hypural the tail of a form of the above length shows 
no essential difference from that of an adult, beyond those 
G 
