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, VV), and the upper and lower lobes of the 
tail becoming pressed backward until their hinder margins 
appose in the axial line.t 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, X, 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, R+/V and Rk +H, must now be 
* Fournal of Morphology, 1X, 1, 1894. 
¢ Gephyrocercy of Ryder. 
