49 



8. — Caitdal Fin and Extremity of Yeetebral 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 (H'p.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 j^art 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, Hj^.'S), 

 and hence the plate {U. + Hp.S) 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 urost3'le 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 

 E 



