408 
MESSRS. P. M. BALFOUR AND W. N. PARKER ON THE 
“ Immediately behind and above this anterior hypural apophysis (as it may be 
termed) is another very much smaller vertical cartilaginous plate, -which may be called 
the posterior hypural apophysis.” 
We have seen that Mivart expresses himself doubtful on the subject. Gegenbaur* 
appears to regard them as haemal arches. 
The latter view appears to us without doubt the correct one. An examination 
of the tail of normal Teleostei shows that the fin-rays of that part of the caudal fin 
which is derived from the ventral lobe of the larva are supported by elements serially 
homologous with the haemal arches, but in no way homologous with the interspinous 
bones of the anal fin. The elements in question formed of cartilage in the larva, 
become ossified in the adult, and are known as the hypural bones. They may appear 
in the form of a series of separate haemal arches, corresponding in number with the 
primitive somites of this region, which usually, however, atrophy in the adult, or more 
often are from the first imperfectly segmented, and have in the adult the form of two 
or three or even of a single broad bony plate. The transitional forms between this 
state of things and that, for instance, in Lepidosteus are so numerous, that there can 
be no doubt that even the most peculiar forms of the hypural bones of Teleostei are 
simply modified haemal arches. 
This view of the hypural bones is, moreover, supported by embryological evidence, 
since Aug. Muller! (p. 205) describes their development in a manner which, if his 
statements are to be trusted, leaves no doubt on this point. 
There are a considerable number of Fishes which are not provided with an obvious 
caudal fin as distinct from the remaining unpaired fins, i.e., Chimaera, Eels, and various 
Eel-like forms amongst Teleostei, and the Dipnoi. Gegenbaur appears to hold that 
these Fishes ought to be classed together in relation to the structure of the caudal 
portion of their vertebral column, as he says on p. 431 of his ‘Comparative Anatomy’ 
(English translation): “ In the Chimterse, Dipnoi, and many Teleostei, the caudal 
portion of the vertebral column ends by gradually diminishing in size, but in most 
Fishes, &c.” 
For our purpose it will, however, be advisable to treat them separately. 
The tail of Chimsera appears to us to be simply a peculiar modification of the 
typical Elasmobranch heterocercal tail, in which the true ventral lobe of the caudal 
fin may be recognised in the fin-fold immediately in front of the filamentous portion 
of the tail. In the allied genus Callorliyncus this feature is more distinct. The 
filamentous portion of the tail of Chimsera constitutes, according to the nomenclature 
adopted above, the true dorsal lobe, and may be partially paralleled in the filamentous 
dorsal lobe of the tail of the larval Lepidosteus (Plate 21, fig. 16). 
The tail of the eel-like Teleostei is again undoubtedly a modification of the 
* * Elements of Comparative Anatomy.’ (Translation), p. 431. 
f “ Beobaclrbnngen zur vergl. Anat. d. Wirbelsaule.” Muller’s Arcliiv., 1853. 
