102, DEAN. Vor LX: 
Diphycercy in its existing conditions, with radials developed, 
as in Ceratodus, is, in the opinion of the present writer, 
a specialized, perhaps more strictly a degenerate condition, 
directly comparable with gephyrocercy, as shown in Echzodon, 
figured by Ryder.! 
On the side of embryology confirmation as to the antiquity 
of heterocercy is singularly clear, if the question of continuous 
dermal fold and of larval fin hair-rays be placed aside. Certain 
it is that the cartilage tail supports, radials, occur first on 
the ventral side, and have here increased to a remarkable 
size, often fusing, before the epural supports come to be 
formed.2 The stimuli that give rise to this outgrowth of the 
caudal lobe have been closely followed by Ryder,® whether 
or not we accept his views as to the exact manner of 
causation. That the tendency was from the earliest towards 
heterocercy is seen in the primitive outgrowth of the lower 
lobe of the tail, and in the consequent upturning of the fin 
end of the notochord. That epurally this axis became 
strengthened by variously grouped neural plates may clearly 
be seen in the embryos of flounder, salmon, shad, Amia or 
Lepidosteus.2?_ It is further noteworthy that the hypural 
border of the upper tail lobe tends for a long while to remain 
rayless, as is well seen in the young of Lepidosteus (or of 
flounder*). The free tip of the chord at a later stage is known 
to become regularly reduced, surrounded by growing and 
fusing radials or basals, or, in the case of Chimaera, curiously 
filamentous. It is, perhaps, significant that there seems in 
every case a stage in development when the heterocercal tail 
suggests a forking character, though this stage may be quickly 
outgrown and masked by a bending together of the lobes.® 
In one of the stages in the development of the flounder‘ 
would be represented the actual condition of Cladoselache, if 
1 Ref. 21, p. 1098, Pl. VIII, Fig. 3. 
2 Gfo lic. .els. 1, Til LV: andi. 
S15 C:, ps LO57- 
Ref. 21, Pll, Fis. 7; 
5 Balfour and Parker, Phil. Trans., Pl. II, 1882, p. 408. 
6 Ref. 21, Pl. 1, Pigs. 7,'8,/9. 
71.c, Pl. I, Fig. 7. 
