1890.] The Homologies of the Fins of Fishes. 407 
from those of the Dipnoi. Huxley,” in 1876, concurred in this 
view of the archipterygium, but regarded the main stem as pass- 
ing through the fifth finger instead of through the thumb, an ar- 
rangement which Gegenbaur has adopted in his second edition. 
(Fig. 4.) 
As has been shown, the view propounded by Mr. Thacher 
in 1877,” and also independently worked out by Prof. Balfour, 
namely, that the paired fins of all vertebrates are formed from a con- 
tinuous lateral fold, corresponding to the median fold from which 
the vertical fins of fishes are derived, is that which is supported 
by the greater weight of evidence, and is now most generally 
adopted. It is strongly supported by the extinct genus of Ac- 
anthodean fishes, Climatius, in which a series of spines extends 
on each side of the abdomen from the pectoral to the ventral fins. 
And these spines are said to be similar to those which support 
the anterior border of these fins. The evidence of paleontology 
is, however, equally in favor of the unibasal paired fin (archiptery- 
gium) as the ancestral form, as it characterized the oldest Elasmo- 
branchii (Ichthyotomi) and Crossopterygia, while the pluribasal 
type is of later age in both divisions of fishes. But it may have 
been derived from a continuous fin, as suggested by Climatius. 
If this view be followed, the Dipnoan paired fins must be re- 
garded as a derivative from the paired fins of the Ichthyotomi, 
which must still be looked upon as the form from which the 
limbs of the Stapedifera or air-breathing vertebrates have been de- 
rived. Baur believes that the latter have been constructed from 
a simple axis like that of Lepidosiren by a process of budding, 
and not by rearrangement of the branches of a fin of the Cerato- 
dus type. The Teleostomata, although specialized in many ways, 
stand on one side of the upward line that leads to the reptiles, 
birds, and mammals, and their paired fins retain much of the 
primitive character. 
2. THE RELATIONS OF FINS TO EACH OTHER. 
The paired limbs of the higher vertebrates appear, at first 
sight, exceedingly unlike the fins of fishes, yet it has long been 
16 Proc. Zodl. Soc. Lon., 1876, Pt. 1, on Ceratodus forsterii. 
1T Proc. Conn. Acad., Vol. II., p. 281. 
