246 VERTEBRATE ANIMALS. 
two pairs of limbs, but these are in the form of awl-shaped organs, 
each supported by a single-jointed cartilaginous rod. The 
pectoral limbs have a membranous fringe inferiorly, and the 
ventrals are placed very far back. There is also a median fin 
behind, forming a continuous fringe round the compressed tail, 
and supported by cartilaginous rays. 
The skull is composed of distinct bones, and there is a lower 
jaw ; but the notochord is persistent, and there are no bodies 
of vertebree developed. ‘The respiratory organs are ¢wo/fold, 
consisting, firstly, of free filamentous branchiz or gills, con- 
tained in a branchial chamber, which opens externally by a 
single vertical gill-slit ; and, secondly, of true lungs, in the form 
of a double cellular air-bladder communicating with the gul- 
let by means of an air-duct or windpipe. Sometimes, if not 
always, there are rudimentary evterna/ gills as well, placed on 
the side of the neck. The heart consists of a ventricle, and of 
Fig. 132.—Lepidosiren annectens, the Mud-fish. £ Pectoral limbs; ¥ Ventral limbs. 
zwo auricles, divided from one another by an incomplete parti- 
tion. Lastly, the nasal sacs open behind into the throat, and 
do not form close chambers opening only by the nostrils, as 
they do in all other fishes except the Myxinoids. The two best- 
known species are the Lepzdosiren faradoxa from the Amazon, 
and the Z. axneclens from the Gambia. They both inhabit 
marshy tracts, and both appear to be able in the dry season to 
bury themselves in the mud, and to form a kind of chamber, in 
which they remain dormant till the rains of the wet season set 
them free. 
Recently there has been discovered in the rivers of Queens- 
land (Australia) a singular fish, which has been referred to this 
order, under the name of Ceratodus Fosteri. It is known to the 
natives as the “ Barramunda,” and attains a length of from three 
to six feet. The body is covered with large cycloid scales, and 
the tail is homocercal. This fish presents many remarkable 
point of resemblance to some curious fossil forms. 
