140 



Mr. R H. Whitehouse. 



[Oct. 30, 



Act. 



Text-fig. 3. — Caudal fin of Trigia lineata as seen after clearing. 

 d.c.r., dorsal caudal radial ; 

 hypural ; 

 l.v., last vertebra ; 

 n.a., neural arch ; 

 .y).c., spinal cord ; 

 7(7:, uiostyle. 



The above figure shows the structure of the endoskeleton as it appears after 

 the bones have been cleared in xylol or turpentine, for, only by so doing can 

 the exact structure be followed. 



Now by comparing tlie features here figured with those of Clupca, it will be 

 seen in what direction specialisation has been at work. (1) The whole 

 of the exserted notochord has beau aborted ; this implies considerable 

 abbreviation. (2) The urostyle is no longer a well-marked independent 

 structure, but is reduced almost to extinction, and lias become incorporated 

 with a hypural bone ; tliis is one of the most important effects of specialisation. 

 (3) Another point of difference from the preceding type is the presence of 

 four hypurals, in tlie place of seven, to the last vertebral segment ; concentra- 

 tion has occurred, the smaller number being most probably due to the lateral 

 fusion of several ; this reduction in the number of hypurals to the last 

 vertebral segment has the effect of producing much greater rigidity and hence 

 usefulness. (4) Although not an invariable rule, a reduced neural arch to 

 the penultimate vertebra, as exhibited in 'frujla, is usually associated with a 

 specialised homocercal caudal fin ; no satisfactory explanation of this can as 

 yet be given. 



(c) Lophius pimUorius (Prdiculati). — An extreme type of specialised 

 homocercy is illustrated by the caudal fin of /.opliim. 



