LUNG-FISHES. 459 



Sub-class I, ELASMOBRANCHII. Sharks and Rays. 

 ,, II. HOLOCEPHALI. Chimseroids. 

 ,, III. DIPNOI. Lung-fishes. 

 IV. TELEOSTOMI. Ganoids and Bony Fishes. 

 Order i. CROSSOPTERYGII. Fringe-filmed Group. 

 ,, ii. ACTINOPTERYGII. Fan-fiimed Group. 

 Section 1. Chondrostei. Ganoids. 

 2. Teleocephali. Bony Fishes. 



Even this system has, however, certain disadvantages, and the following 

 arrangement appears preferable. This is as follows, viz. : 



Sub-class I. DIPNOI. Lung-fishes. 



,, II. HOLOCEPHALI. Chimseroids. 

 ,, III. TELEOSTOMI. Bony Fishes and Ganoids. 

 Order i. ACTINOPTERYGII. Fan-finned Group. 



ii. CROSSOPTERYGII. Fringe-finned Group. 

 Sub- class IV. EXASMOBRANCHII. Sharks and Rays. 



With these preliminary remarks, we may proceed to a brief systematic 

 review of the various groups of the class. It must, however, be remembered 

 that the number of existing fishes is so vast that it is quite impossible to 

 notice even all the family types, while of the genera only a small percentage 

 are selected for mention. It must also be born in mind that the true relation- 

 ships of the sub-classes cannot be properly grasped unless the fossil forms 

 are also studied. 



SUB-CLASS I. DIPNOI. 

 LUNG-FISHES. 



LIKE the next, the present group is now represented by a very small 

 number of species, and as it was abundant in earlier epochs, it is evidently 

 one that is on the wane. Whereas in many respects the lung-fishes approxi- 

 mate to the amphibians, in others they show affinity to the earlier sharks ; 

 and it has been suggested that they ought to form a class intermediate 

 between Amphibia and Pisces. Regarding them, however, merely as a sub- 

 class of the latter, they agree with the Chimseroids in the essential feature 

 that there is no movable separate suspending arrangement for the lower jaw ; 

 and from this peculiarity the name of Autostylica has been suggested for the 

 two sub-classes in question. In the existing members of the group, which 

 form the order Sirenoidei, the body is thick and spindle-shaped, its covering 

 consisting of large overlapping horn-like scales, and the tail forming a diphy- 

 cercal fin. In form, the head approximates to that of amphibians, as it does in 

 its slimy integument. The paired fins are of the fringed type, their skeleton 

 consisting typically of a jointed cartilaginous rod, fringed on both sides with 

 divergent rays. In the skull the original cartilaginous cranium is roofed ove'r 

 with dermal membrane-bones, and the nostrils have posterior apertures 

 opening into the back of the mouth to form true posterior nostrils, the latter 



