268 CLASSIFICATION OF VERTEBRATES. 



testine ; a cloaca present, air-bladder single or paired, function- 

 ing as a lung. 



The dipnoi, or lung-fishes, are frequently regarded as belong- 

 ing to the ganoids, and have attracted great attention from the 

 fact that they are often considered as intermediate in position 

 between the lower fishes and the amphibia. In past time the 

 group was richly represented ; but in the existing fauna of the 

 earth but four species are known, these having that wide and 

 discontinuous distribution so frequently characteristic of the 

 survivors of an ancient group. 



They all have an elongate fish-like or eel-like body, covered 

 in the recent species with overlapping cycloid scales ; while in 

 the fossils ganoid scales frequently occurred. The fins are sup- 

 ported by horny dermal rays. The axial skeleton consists of 

 the persistent notochord, around which (except in the tail region 

 of some forms) vertebral centra are not developed, segmenta- 

 tion being shown only in the neural arches and ribs. 



FIG. 269. Skull of Protopterus, after Wiedersheim. , angulare ; d, dentary ; 

 e, otic capsule; /, skeleton of fore limb; fp, frontoparietal ; g, external gills; //, 

 hyoid ; ///', head rib; , nasal; nc, nasal capsule; //, palatopterygoid ; y, quadrate; 

 s, supracranial bone ; sg, squamosal ; so, supraoccipital ; /- I 'I, branchial arches. 



The skull consists of the largely persistent chondrocranium 

 plus a number of membrane bones not easily homologized with 

 those of other vertebrates. In the chondrocranium exoccipitals 

 alone are developed. In the lower groups the membrane bones 

 are very numerous ; but in existing forms, as in the extinct 



