DIPNOI. 425 



arch has a clavicle ; but the scapular arch is attached to the 

 occiput. In Ceratodus the pectoral and ventral limbs have the 

 same form as in the Crossopterygious Ganoids. The hinder 

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

 heart has two auricles and one ventricle. The respiratory 

 organs are twofold, consisting, on the one hand, of free fila- 

 mentous 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, com- 

 municating with the oesophagus by means of an air-duct or 

 trachea. The branchiae are supported 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. 



If these characters are examined a little more minutely, it is 

 easy to point to those in which the Lepidosiren approaches the 

 Fishes, and to those in which it resembles the Amphibians. 

 It resembles the Fishes in the shape of the body, and in the 



Fit;. 182. Dipnoi. Lepidosiren annectens. 



possession of a covering of horny overlapping scales of the true 

 cycloid character ; whilst the limbs are more tike those of fishes 

 than of reptiles. The fin, also, which clothes the posterior 

 extremity of the body, is of a decided fish-like character. The 

 most marked piscine feature, however, is the presence of free 

 branchiae, attached to branchial arches, and placed in a branchial 

 cavity, which opens internally into the pharynx by a number of 

 slits, and communicates externally with the outer world by 

 means of a single vertical gill-slit. 



On the other hand, the Lepidosiren approximates to the 

 Amphibians in the following important points : The heart 

 consists of three cavities, two auricles and a single ventricle. 

 True lungs are present, with a trachea and glottis, returning 

 their blood to the heart by a distinct pulmonary vein, and in 

 every respect discharging the functions of the lungs of the 

 higher Vertebrates. It is true that the lungs of the Lepidosiren 

 are merely a modification of the swim-bladder of the other 

 fishes, but the significance of the change of function is not 



