CHAPTER II. 



... orb. 



The Lung-Fishes and Chim^eroids, — 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- 

 fishes 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 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. ) 



