410 
MESSRS. E. M. BALFOUR AND W. N. PARKER ON THE 
We have had opportunities of examining the structure of the tail of Ceratodus and 
Protopterus in dissected specimens in the Cambridge Museum. The vertebral axis 
runs to the ends of the tail without showing any signs of becoming dorsally flexed. 
At some distance from the end of the tail the fin-rays are supported by what are 
apparently segmented spinous prolongations of the neural and haemal arches. The 
dorsal elements are placed above the longitudinal dorsal cord, and occupy therefore 
the same position as the independent elements of the neural arches of Lepidosteus . 
They are therefore to be regarded as homologous with the dorsal fin-supports or 
interspinous bones of other types. The corresponding ventral elements are therefore 
also to be regarded as interspinous bones. 
In view of the fact that the fin-supports, whenever their development has been 
observed, are found to be formed independently of the neural and haemal arches, we 
may fairly assume that this is also true for what we have identified as the interspinous 
elements in the Dipnoi. 
The interspinous elements become gradually shorter as the end of the tail is 
approached, and it is very difficult from a simple examination of dissected specimens 
to make out how far any of the posterior fin-rays are supported by the haemal arches 
only. To this question we shall return, but we may remark that, although there is a 
prolongation backwards of the vertebral axis beyond the last interspinous elements, 
composed it would seem of the coalesced neural and haemal arches but without the 
notochord, yet by far the majority of the fin-rays which constitute the apparent caudal 
fin are supported by interspinous elements. 
The grounds on which we hold that the tail of the Dipnoi is to be regarded as a 
degenerate rather than primitive type of tail are the following :— 
(1) If it be granted that a diphycereal or protocercal form of tail must have 
preceded a heterocercal form, it is also clear that the ventral fin-rays of such a tail 
must have been supported, as in Polypterus and Calamoicthys, by haemal arches, and 
not by interspinous elements; otherwise, a special ventral lobe, giving a heterocercal 
character to the tail, and provided with fin-rays supported only by haemal arches, 
could never have become evolved from the protocercal tail fin. Since the ventral fin- 
rays of the tail of the Dipnoi are supported by interspinous elements and not by 
haemal arches, this tail fin cannot claim to have the character of that primitive type of 
diphycercal or protocercal tail from which the heterocercal tail must be supposed to 
have been evolved. 
(2) Since the nearest allies of the Dipnoi are to be found in Polypterus and the 
Crassopterygidae of Huxley, and since in these forms (as evinced by the structure of 
the tail fin of Polypterus , and the transitional type between a heterocercal and diphy¬ 
cercal form of fin observable in fossil Crassopterygidse) the ventral fin-rays of the 
caudal fin were clearly supported by lisemal arches and not by interspinous elements, 
it is rendered highly probable that the absence of fin-rays so supported in the Dipnoi 
is a result of degeneration of the posterior part of the tail. 
