CHAPTER II. 
The Lung-Fishes and Chimh:roids,— Subclasses 
Dipnoi and Holocephali. 
The two first subclasses of fishes agree with one another, and thereby differ from 
the remaining two, in the structure of the skull, in which the hyomandibular bone 
is welded with what is known as the palatopterygoid bar (that is to say, the bones 
corresponding with the palatines and pterygoids of the higher Vertebrates), which 
is itself firmly united to 
the cranium proper, so 
that there is no separate 
structure for the suspen¬ 
sion of the lower jaw. 
To this type the name of 
solid-skulled (technically, 
autostylic) fishes may be 
applied; and it may be 
noted that this type of 
structure is essentially the 
same as that on which the 
skulls of the Amphibians 
are formed. In the lung- 
ffshes the skeleton is par¬ 
tially ossified, with well- 
developed membrane- 
bones ; the gill-clefts are 
but slightly separated, and 
open into a single cavity 
protected by an external 
cover; and the external 
skeleton consists of true 
bony tissue. In the 
existing 1 members of the 
group the optic nerves (or 
those proceeding from the 
brain to the eyes) simply 
cross one another, without any interlacing of the constituent fibres, the intestine 
has a spiral valve; the air-bladder is elongated, and performs the functions of a 
lung; and the nostrils open posteriorly by two apertures into the cavity of the 
ROOF OF THE SKULL OF THE AUSTRALIAN LUNG-FISH. 
A, anterior, and B, posterior median plate ; C, inner, and D, outer lateral 
plate ; na, nostrils ; orb, socket of the eyes. (From Teller.) 
