ORDERS OF FISHES. 



165 



is defined by the following characters : The lody is fish- 

 like in shape. There is a skull loith distinct cranial hones 

 and a loioer jaw, hut tlie notochord is persistent, and there are 

 no vertebral centra, nor an occipital condyle. The exoskele- 

 ton consists of horny, overlapping scales, having the " cycloid " 



Fig. 533. — Dipnoi. Lepidosiren annectens. 



Fig. 534. 



The Austnilian Mud-flsh. 



character. The pectoral and ventral limhs are hoth present, hut 

 have {in Zepidosiren) the form of awl-shaped, filiform, many- 

 jointed organs, of which the former only have a membranous 

 fringe inferiorly ; whereas in other cases {Geratodus) they re- 

 semble these organs in the Crossopterygious Ganoids. The ventral 

 limhs are attached close to the anus, and the pectoral arch has a 

 clavicle; but the scapular arch is attached to the occiput. The 

 hinder extremity of the body is fringed by a vertical median fin. 

 The heart has two auricles and one ventricle, in Lepidosiren, 

 but consists of an auricle, ventricle, and arterial bulb in Gera- 

 todus. The respiratory organs are twofold, co7isisting on the one 

 hand, of free filamentous gills contained in a branchial chamber, 

 which opens externally by a single vertical gill-slit ; and on the 

 other hand of true lungs in the form of a double cellular air- 

 bladder, communicating with the oesophagus by means of an air- 

 duct or trachea. The branchicB are suppoi'ted upon branchial 

 arches, but these are not connected with the hyoid bone ; and in 

 some cases, at any rate, rudimentary external branchiae exist as 

 well. The nasal sacs open posteriorly into the throat. 



Until lately the only known members of the order Dipnoi 

 were the -Lepidosiren paradoxa of South America and the 



