THE RELATIONSHIPS OF FISHES. 527 



ovaries are strikingly like those of Amphibians ; the oviduct seems to 

 be the Miillerian duct. Ureters and genital ducts open beside one 

 another into the cloaca. 



Lepidosiren. — Relatively little is known in regard to the third type, 

 Lepidosiren, from the Amazons. It has an eel-shaped body, with 

 a continuous vertical fin. The limbs are reduced to cylindrical stems, 

 without any radials. There are no external gills. The air-bladder or 

 lung is double, and its relations to blood vessels are like those in 

 Protopterus. 



There is an imperfect muscular septum dividing the auricle into two, 

 and there is a similarly incomplete septum in the ventricle. The conus 

 resembles that of Protopterus. 



The relationships of fishes. — Balfour regarded the Elasmo- 

 branchs as nearest the ancestral stock ; while from hypothetical Proto- 

 Ganoids he derived on the one hand the Dipnoi, on the other hand the 

 Ganoids, and thence the Teleosteans. 



But it must be noted that the Dipnoi are markedly separated in many 

 ways from living Ganoids. Moreover, the extinct Ganoids form a very 

 large and diverse series, which cannot be fairly appreciated by a study 

 of the few survivors. Nor does the palaeontological evidence bear out 

 the separateness of Teleosteans and Ganoids. 



Gadow proposes the following arrangement : — 



Class Ichthyes. 



I. Sub-class Pisces. 



I. Division, Elasmobranchii. 



1. Order Proselachii, e.g. Plewacanthus. 



2. Order Plagiostomi, including Selachii and Raise. 

 II. Division Acanthodi. 



III. Division Holocephali. 



TV TV ' • ( *• Order Oossopterygii, 



TelmVtomi' \ e.g. Polyfterus. < (a) Chondrostei (Sturgeon, etc.). 



leleostomi. ^ ^ Order Actinopterygli. - («) Holostei (Lepidosteus, etc.). 



( (c) Teleostei (Bony Fishes). 



II. Sub-class Dipnoi. 



[Table. 



