No. 1.] MORPHOLOGY OF CLADOSELACHE. IOI 
diphycercy, have carefully reviewed this primitive condition 
from the standpoint of the palaeontological material then 
extant. Ryder?! in 1884, treating the subject of fish fins 
mainly from the embryological side, has given a most 
important memoir upon the evolution of the tail of fishes, 
comparing his results critically with those of Kélliker, Vogt, 
Dohrn and Lotz. He brings out clearly the degenerate stage 
in the development of the tail, whereby the homocercal is 
reduced to a diphycercal type which he terms gephyrocercy. 
Reviewing the matter carefully, in view of the puzzling 
caudal structures of Cladoselache, the present writer is led 
to suggest that the origin of the caudal of gnathostome fishes 
is to be derived not through a diphycercal, but through a 
heterocercal condition. He believes, furthermore, that embryo- 
logical results might in this way be interpreted. That phylo- 
genetically the heterocercal type itself may have been evolved 
from some form of diphycercy, wanting, however, in radials, 
would appear extremely probable. 
From the standpoint of palaeontology, there can certainly 
be little doubt that the heterocercal condition can easily 
claim priority in time, — heterocercy is represented in 
all Elasmobranchs (Xenacanthids excepted), in all known 
Ostracoderms. In recent forms it maintains in chondrostean 
Teleostomes, either in adult or young, and in bony teleostomes 
as a regular embryonic condition. The only fish group 
whereby true diphycercy can claim antiquity among fossil 
forms is the Dipnoan, since the condition of caudal in 
Crossopterygians is generally heterocercal in the most ancient 
types, Holoptychius, Osteolepis. 
If the fins of Xenacanthids are reduced to selachian con- 
ditions, why should not the tail, in view of its homologous 
origin, be regarded as also derivable from a selachian tail type? 
If dipnoan-like fins in this group are secondarily acquired, why 
should not also the dipnoan tail structures? If it is shown 
that the lateral fins of Xenacanthids may be reduced to the 
more ancient and more primitive type of Cladoselache, the 
tail structure in these forms, simple though it appears, might 
equally well be regarded as of acquired character. 
