A R THR ODIRAN L UNG-FISHES 



129 



The advancing structures of the Dipnoan include, in 

 addition : - 



I. EXOSKELETAL SPECIALIZATIONS. Head-roofing der- 

 mal bones (cf., however, Pleuracanthid) and cycloidal scales. 

 In early forms (Dipterus) these appeared at the surface 

 and were apparently enamelled. In recent forms they 

 are deeply sunken in the integument (Prototerus). They 

 suggest closely the structures of Crossopterygian (p. 149). 



II. ARTICULATION OF THE MANDIBLE. This is auto- 

 stylic, somewhat as in Chimaeroid (v. p. 256). Its homol- 

 ogy is obscure. 



III. AIR-BLADDER, (v. p. 264). 



IV. ABSENCE OF VENTRAL "CLASPERS" (cf., however, 

 Cladoselache). 



V. TRUE POSTERIOR NARES (amphibian). 



VI. THE GREAT SIZE OF THE CELLULAR ELEMENTS OF 



ALL TISSUES (amphibian) ; THE GLANDULAR STRUCTURES 

 OF THE EPIDERMIS (amphibian). 



VII. CIRCULATORY CHARACTERS : the three-chambered 

 heart ; aortic arches. 



VIII. LIMB STRUCTURE. This, however, is not to be 

 interpreted as in any way directly transitional to cheirop- 

 terygium. 



The Arthrodiran Lung-fishes 



The ARTHRODIRA, as Smith Woodward has shown, may 

 provisionally be regarded as an order of extinct and highly 

 specialized lung-fishes. They occur geologically among the 

 earliest fishes, and include a number of (Devonian) forms 

 whose peculiar characters and gigantic size must have made 

 them among the most striking members of ancient fauna. 

 The group might be regarded as standing in the same rela- 

 tion to the ancient Dipnoans as Acanthodians to the Cla- 



K 



