400 MANUAL OF ZOOLOGY. 



resemblance between the two, that until recently the Lepidosiren 

 (fig. 152) was always made to constitute the lowest class of 

 the Amphibia. The highest authorities, however, now concur in 

 placing it amongst the fishes, of which it constitutes the highest 

 order. The order Dipnoi is defined by the following char- 

 acters : — The body is fish-like in shape. There is a skull with 

 distinct cranial bones and a lower jaw, but the notochord is 

 persistent, and there are no vertebral centra, nor an occipital 

 condyle. The exoskeleton consists of small, homy, overlap- 

 ping scales, having the "cycloid" character. The pectoral 

 and ventral limbs are both present, but have the form of awl- 

 shaped, filiform, many-jointed organs, of which the former only 

 have a membranous fringe inferiorly. The ventral limbs are 

 attached close to the anus, and the pectoral arch has a clavicle; 

 but the scapular arch is attached to the occiput. The hinder 



Fig, 152. — Dipnoi. Lepidosiren annectens. 



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 hyoidbone; and in some 

 cases,, at any rate, rudimentary «/«^«a/ 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 

 possession of a covering of horny overlapping scales of the true 

 cycloid character; whilst the limbs are more like 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 Dlaced in a branchial 



