STRUCTURE ART) DEVELOPMENT OF LEPIDOSTEUS. 
411 
[We use this argument without offering any opinion as to whether the diphycercal 
character of the tail of many Crassopterygidae is primary or secondary.] 
(3) The argument just used is supported by the degenerate and variable state of 
the end of the vertebral axis in the Dipnoi—a condition most easily explained by 
assuming that the terminal part of the tail has become aborted. 
(4) We believe that in Ceratodus we have been able to trace a small number of the 
ventral fin-rays supported by haemal arches only, but these rays are so short as not to 
extend so far back as some of the rays attached to the interspinous elements in front. 
These rays may probably be interpreted, like the more or less corresponding rays in 
the tail of the Eel, as the last remnant of a true caudal fin. 
The above considerations appear to us to show with very considerable probability 
that the true caudal fin of the Dipnoi has become all but aborted like that of various 
Teleostei; and that the apparent caudal fin is formed by the anal and dorsal fins 
meeting round the end of the stump of the tail. 
From the adult forms of Dipnoi we are, however, of opinion that no conclusion can 
be drawn as to whether their ancestors were provided with a diphycercal or a hetero- 
cercal form of caudal fin. 
The general conclusions with reference to the tail fin at which we have arrived are 
the following:—• 
(1) The ventral lobe of the tail-fin of Pisces differs from the other unpaired fins in 
the fact that its fin-rays are directly supported by spinous processes of certain of the 
haemal arches instead of independently developed interspinous bones. 
(2) The presence or absence of fin-rays in the tail fin supported by haemal arches 
may be used in deciding whether apparently diphycercal tail fins are aborted or 
primitive. 
Excretory and Generative Organs. 
I.— Anatomy. 
The excretory organs of Lepidosteus have been described by Muller (No. 13) and 
Hyrtl (No. 11). These anatomists have given a fairly adequate account of the 
generative ducts in the female, and Hyrtl has also described the male generative 
ducts and the kidney and its duct, but his description is contradicted by our obser¬ 
vations in some of the most fundamental points. 
In the female example of 100'5 centims. which we dissected, the kidney forms a 
paired gland, consisting of a narrow strip of glandular matter placed on each side of the 
vertebral column, on the dorsal aspect of the body cavity. It is covered on its ventral 
aspect by the oviduct and by its own duct, but is separated from both of these by a 
layer of the tough peritoneal membrane, through which the collecting tubes pass. It 
extends forwards from the anus for about three-fifths of the length of the body-cavity, 
and in our example had a total length of about 28 centims. (Plate 26, fig. 60, h). 
