100 DEAN. [Vou. IX. 
margin was slightly crenulate, and bears a coarser type of 
granular shagreen than that which covered the entire tail. 
The tail’s anterior margin, like that of the other fins, was 
coarsely shagreened. 
There can be little doubt that on either side of the base 
of the tail there was present a longitudinal derm fold or keel, 
not unlike that developed in a number of recent fish forms 
whose caudal outline is similar to that of Cladoselache. In 
specimens whose ventral aspect is preserved (and these are 
in the majority of instances) the flattened body terminal, 
exaggerated doubtless by mechanical causes, spreads out like 
the fluke of a sirenian, the tail itself to be seen in vertical 
projection as an acutely produced apex; the outline has in 
these instances been formed by the lateral keels. That this 
keel was a stout one appears probable, as traces of it are to be 
found in examples which have preserved the lateral aspect of 
the tail. 
The significance of the caudal of Cladoselache is not readily 
to be determined. At first sight its apparent specialization is 
not at all in keeping with the peculiarly archaic and generalized 
character of the paired fins. Its broadly heterocercal tail 
has appeared to have reached almost the limits of homocercy, 
— it certainly seems more specialized than the caudal of 
Xenacanthids. The peculiar nature, on the other hand, of the 
epural supporting plate, and of the dermal margin of the upper 
lobe leads one naturally to the closest examination of suggested 
relationships, homodynamous, to paired fins. 
The type in this early fin is certainly a pure heterocercal 
one, in no way approaching the archaic diphycercy which one 
might naturally expect in the earlier kindred of Xenacanthus. 
Diphycercy as represented in Ceratodus has been (as far as 
the writer is aware) generally regarded as the most primi- 
tive condition of the body terminal of aquatic vertebrates.} 
McCoy 19 in 1848 and Cope 2° in 1871, in their comments on 
1 Since the above was written, the memoir of W.N. Parker on Protopterus 
(Trans. Roy. Irish Acad., 1892) has been received. His view in this matter, 
strengthening an earlier influence of Dr. Traquair (Brit. Ass. Rep., 1871), is that 
“it is impossible to draw any conclusions with regard to the ancestral form of 
caudal fin in the Dipnoi from a study of adult forms.” 
