CHAPTER II 



, orb. 



THE LUNGFISHES AND CHIM/EROIDS SUBCLASSES DIPNOI 

 AND HOLOCEPHALI 



THE; first two 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 Vertebates) , 

 which is itself firmly 

 united to the cranium 

 proper, so that there is no 

 separate structure for the 

 suspension of the lower 

 jaw. To this type the 

 name of solid - skulled 

 (technically autostylic) 

 fishes maybe applied; and 

 it may be noted that this 

 type of structure is essen- 

 tially the same as that on 

 which the skulls of the 

 Amphibians are formed. 

 In the lungfishes the 

 skeleton is partially ossi- 

 fied, 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 con- 

 sists of true bony tissue. 

 In the existing members 

 of the group the optic 

 nerves (or those proceed- 

 ing 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 mouth, after the 



(2693) 



ROOF OF THE SKULL OF THE AUSTRALIAN LUNGFISH. 



A. anterior, and B. posterior median plate; C. inner, and D. outer lateral 



plate; na. nostrils; orb. socket of the eyes. 



(From Teller.) 



