[We use this argument without offering any opinion as to whether the diphycercal character of the tail of many Crossopterygidæ 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 hæmal 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 shew 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 heterocercal 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 hæmal arches instead of independently developed interspinous bones.
(2) The presence or absence of fin-rays in the tail-fin supported by hæmal arches may be used in deciding whether apparently diphycercal tail-fins are aborted or primitive.
[521] These specimens were given to us by Professor W. K. Parker, who received them from Professor Burt G. Wilder.
[522] Gegenbaur (No. 6) takes a different view on this subject, as is clear from the following passage in this memoir (pp. 369-370):—“Each vertebra of Lepidosteus thus consists of a section of the notochord, and of the cartilaginous tissue surrounding its sheath, which gives origin to the upper arches for the whole length of the vertebral column, and in the caudal region to that of the lower arches also. The latter do not however complete the enclosure of a lower canal, but this is effected by special independent elements, which are to be interpreted as homologues of the ribs.” (The italics are ours.) While we fully accept the homology between the ribs and the lower elements of the hæmal arches of the tail, the view expressed in the italicised section, to the effect that the lower parts of the caudal arches are not true hæmal arches but are independently formed elements, is entirely opposed to our observations, and has we believe only arisen from the fact that Gegenbaur had not the young larvæ to work with by which alone this question could be settled.