154 A. A. W. HUBRECHT. 
carrying certain hereditary stigmata which pointed to the 
fact that once they had been air-breathers already. 
Up to now we only know such a ridiculously small portion 
of all the fossil animals that have lived in the palzozoic 
period, that it is not foolhardy to predict that very numerous 
remains may yet in future be unearthed in which this question 
presents itself. 
And if we think of those innumerable series of species, 
genera, families, and orders of which at present we know 
nothing, is it then improbable that in those earlier periods of 
the world’s history the same phenomenon of a secondary 
return to the aquatic medium has presented itself over and 
over again ? 
If I were allowed to point to one example I would select 
Polypterus, and ask if its paired and ventral air-bladder 
might perhaps not have served as effectual lungs to a more 
fully air-breathing ancestor, and if Klaatsch’s hypothesis 
(96) of the phylogeny of its hmb-skeleton might not easily 
be turned the other way round so that the central plate with 
the two longer bones right and left of it should not be looked 
upon with Klaatsch as an incipient carpus with lateral radius 
and ulna, but as an adaptation of what had already functioned 
as a supporting Jimb-pair in a terrestrial ancestor to a re- 
assumed aquatic life ? 
Similar questions might be put concerning the Dipnoi, 
who in the Devonian ateet appear to have had—judging 
from footprints—five-toed tetrapod contemporaries. Even in 
Teleosts (Saccobranchus and Anabas scandens) evolutionary 
processes are going on even now which tend to an exchange 
of the aquatic for the atmospheric life and vice versa. 
The air-bladder in the 'Teleosts—which by common consent 
is now generally derived from arrangements such as they are 
now possessed e. g. by Polypterus, and not vice versa—has 
this other curious particularity that in certain closely allied 
species of the genera Scomber, Sebastes, Umbrina, Thynnus, 
Chironectes, it may be totally absent in the one, present in the 
other. Thus according to Stannius, ‘Zootomie der Fische,’ 2e 
