AMPHIBIA. 439 
separate and widely apart with no operculum. The skeleton 
is cartilaginous and the palatoquadrate is free from the 
cranium. There is a spiral valve to the intestine. A cloaca 
is present and the kidney is mainly a metanephros. 
Development is purely embryonic, the egg has much yolk 
and the young is not hatched till like the adult. 
The Lvasmobranchit are marine. The sharks are mostly 
pelagic and the skates and rays mainly littoral or katantic. 
OrverR III.—olocephal. 
A small order formed to contain Chimera (the King of the 
Herrings) and its allies. They resemble the last order in 
their cartilaginous skeleton and some other structural features, 
but differ in having an operculum covering the gill-slits, a 
protocercal tail and no cloaca. The palatoquadrate (upper 
jaw) is completely fused to the cranium. 
The few genera are widely scattered, one being a deep- 
sea type. 
ORDER 1V.—Dzpnoz. 
The Dzpnoi or mud-fishes differ from the other orders 
of fishes in the possession of true lungs in addition to their 
gills, and in the partial division of the auricle into two, thus 
producing a three-chambered heart; the nasal sacs have 
internal nares. The paired fins are archipterygia, ze, a 
central axis with rays on each side; the caudal fin is pro- 
tocercal. There is a spiral valve and a cloaca and the 
skeleton.is partly cartilaginous and partly bony. 
Like the more primitive of the Zeleostomi, the Dipnoi 
-are freshwater forms and have a discontinuous distribution. 
Ceratodus is found in Australia, Profopterus in the Nile and 
Lepidosiven in the Amazon. 
Cuass III.—AMPHIBIA. 
The Amp/ibia form a transition class from the two pre- 
ceding to the three following terrestrial classes. The frog is 
about the most terrestrial of all the class. Gills, median 
fins and lateral line sense-organs are found throughout life 
