ARTHRODIRAN LUNG-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 Chimeroid (v. p. 256). Its homol- 
ogy is obscure. 
III. Arr-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. CrrcuLATORY CHARACTERS: the three-chambered 
heart ; aortic arches. 
VIII. Limes 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 
