236 Subclass Dipneusti, or Lung-fishes 
Paleozoic time, their origin with that or through that of the 
latter to be traced to the Ichthyotomi or other primitive sharks. 
These two groups are separated from all the more primitive 
fish-like vertebrates by the presence of lungs. In its origin 
the lung or air-bladder arises as a diverticulum from the ali- 
mentary canal, used by the earliest fishes as a breathing-sac, 
the respiratory functions lost in the progress of further di- 
vergence. Nothing of the nature of lung or air-bladder is 
found in lancelet, lamprey, or shark. In none of the remaining 
groups of fishes is it wholly wanting at all stages of develop- 
ment, although often lost in the adult. Among fishes it is most 
completely functional in the Dipneusti, and it passes through 
all stages of degeneration and atrophy in the more specialized _ 
bony fishes. 
In the Dipneusti, or Dipnoans, as in the Crossopterygians 
and the higher vertebrates, the trachea, or air-duct, arises, as 
above stated, from the ventral side of the cesophagus. In the 
more specialized fishes, yet to be considered, it is transferred 
to the dorsal side, thus avoiding a turn in passing around the 
cesophagus itself. From the sharks these forms are further 
distinguished by the presence of membrane-bones about the 
head. From the Actinoptert (Ganoids and Teleosts) Dipnoans 
and Crossopterygians are again distinguished by the presence 
of the fringe-fin, or archipterygium, as the form of the paired 
limbs. From the Crossopterygians the Dipnoans are most 
readily distinguished by the absence of maxillary and pre- 
maxillary, the characteristic structures of the jaw of the true 
fish. The upper jaw in the Dipnoan is formed of palatal ele- 
ments attached directly to the skull, and the lower jaw con- 
tains no true dentary bones. The skull in the Dipnoans, as 
in the Chimera, is autostylic, the mandible articulating directly 
with the palatel apparatus, the front of which forms the upper 
jaw and of which the pterygoid, hyomandibular and quadrate 
elements form an immovable part. The shoulder-girdle, as 
in the shark, is a single cartilage, but it supports a pair of super- 
ficial membrane-bones. 
In all the Dipnoans the trunk is covered with imbricated 
cycloid scales and no bony plates, although sometimes the 
scales are firm and enameled. The head has a roof of well- 
