DIPNOI. 375 



scale of fishes, to which also the permanently heterocercal tail 

 conduced. When we come, however, to take into consider- 

 ation the sum of all their characters, there can be little hesita- 

 tion in placing the order nearly at the summit of the entire 

 class. The nervous system, and especially the cerebral mass, 

 is very much more highly developed proportionately than is 

 the case with any other division of the fishes. The organs of 

 sense are, comparatively speaking, of a very high grade of or- 

 ganisation, the auditory organs being more than ordinarily 

 elaborate, the eyes being sometimes furnished with a third eye- 

 lid (membrana nictitans), and the nasal sacs having a very com- 

 plex structure. The structure of the heart agrees with that of 

 the Ganoids, and is a decided advance upon the heart of the 

 more typical bony fishes. Finally, the embryo, before its 

 exclusion from the egg, is furnished with external filamentous 

 branchiae, this being a decided approximation to the Amphibia. 

 ORDER VI. DIPNOI (=Protopteri, Owen). This order is a 

 very small one, and includes only the singular Mud-fishes (Lepi- 

 dosiren) ; but it is nevertheless of great importance as exhibit- 

 ing a distinct transition between the fishes and the Amphibia. 

 So many, in fact, and so striking, are the points of resemblance 

 between the two, that until recently the Lepidosiren (fig. 144) 

 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 characters : 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, horny, overlapping 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 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 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 branchiae 

 are supported upon branchial arches, but these are not con- 

 nected with the hyoid bone \ .and in some cases, at any rate, 



