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THE DIPNEUSTA. 213 
has lately been discovered in Australia. During the dry 
season, that is in summer, these strange animals bury 
themselves in a nest of leaves in the dry mud, and then 
breathe air through lungs like the Amphibia. But during 
the wet season, in winter, they live in rivers and bogs, 
and breathe water through gills like fish. Externally, they 
resemble fish of the eel kind, and are like them covered 
with scales; in many other characteristics also—in their 
internal structure, their skeleton, extremities, ete—they 
resemble Fish more than Amphibia. But in certain features 
they resemble the Amphibia, especially in the formation 
of their lungs, nose, and heart. There is consequently an 
endless dispute among zoologists, as to whether the Mud- 
fish are genuine Fish or Amphibia. Distinguished zoologists 
have expressed themselves in favour of both opinions 
But in fact, owing to the complete blending of character- 
istics which they present, they belong neither to the one 
nor to the other class, and are probably most correctly 
dealt with as a special class of Vertebrata, forming the 
transition between Fishes and Amphibians. The still living 
Dipneusta are probably the last surviving remains of a 
group which was formerly rich in forms, but has left no 
fossil traces on account of the want of a solid skeleton. 
In this-respect, these animals are exactly like the Monor- 
rhina and the Leptocardia. However, teeth are found in 
the Trias which resemble those of the living Ceratodus. 
Possibly the extinct Dipneusta of the palzeolithic period, 
which developed in the Devonian epoch out of primeval 
fish, must be looked. upon as the primary forms of the 
Amphibia, and thus also of all higher Vertebrata. At 
all events the unknown forms of transition—from Primeeval 
fish to Amphibia—were probably very like the Dipneusta. 
