38 PRIMITIVE CAUDAL FIN 



been acquired in ancient sharks (Fig. 46). The fin of Fig. 



47, however, has not generally been looked upon as derived 

 from shark-like conditions ; it has, on the other hand, been 

 thought to be most nearly of the ancestral form. The 

 vertebral axis does not appear to be upturned, and the 

 ventral and dorsal lobes of the fin remain nearly sym- 

 metrical, or diphycercal. This form of the caudal fin, on 

 the other hand, has been noted to present many degener- 

 ate characters, and to the writer * it seems more reasona-_ 



.ble to regard the diphycercal condition as in many cases 

 directly descended from the heterocercal. This might be 

 effected by the terminal portion of the vertebral rod abort- 

 ing (as in Fig. 47, N), and the upper and lower lobes of the 

 tail becoming pressed backward until their hinder margins 

 appose in the axial line.f The form of diphycercy which 

 is seen in Fig. 119 is unquestionably of little morphological 

 value ; it occurs commonly in deep-sea fishes of every group, 

 and must be looked upon as a degenerate condition result- 

 ing from impeded motion under the conditions of bathyb- 

 ial, or deep-sea living. 



The cartilaginous supports of the caudal, like those of 

 other unpaired fins, become greatly reduced in size by the 

 encroachment of dermal rays. In the tail of the fossil 

 shark (Fig. 46) the cartilaginous supports, R, extend to the 

 very margin of the fin : in the modern shark (Fig. 45) a 

 large part of the functional fin area has become of second- 

 ary, or dermal origin, D. In the caudals of Figs. 47 and 



48, distinct dermal rays, D, are seen, extending from the 

 body wall to the fin margin, splitting and segmenting dis- 

 tally in becoming more perfectly specialized in function. 

 The cartilaginous supports, 7? + .Vand R + H, must now be 



* Journal oj Morphology, IX, I, 1894. 

 f Gephyrocercy of Ryder. 



