XX] MAIN DIVISIONS 511 



conspicuous of which form a paired series between which a median 

 series is beginning to appear (as happens also in Acipenser). We 

 conclude therefore that these differences between Amphibia and 

 modem Dipnoi are due to degeneration on the part of the latter 

 and are not primitive. The gill-arches of modern Dipnoi are simple 

 rods with at most a single joint, as in Amphibia, and the bones of 

 the opercular flap are represented by a single bone which strongly 

 resembles the squamosal in Amphibia in position. 



The body is covered with thin scales deeply sunk in the skin. 

 These scales have a well developed bony layer covered with dentine. 

 The scales of extinct Dipnoi resembled those of Osteolepidoti in 

 possessing a superficial layer of pulp-cavities covered with a thin 

 layer of ganoin. The scales of both recent and fossil fish agree in 

 having little solid spines covering their margins, not to be con- 

 founded with placoid denticles such as cover both margins of the 

 scales of Lepidosteus, Polypterus and some Siluroid fish. Modem 

 Dipnoi are divided into two suborders, viz. : 



(1) Monopneumona, including Ceratodus, characterised by the 

 undivided air-bladder, the well- developed archipterygial paired fins, 

 the hyoidean pseudobranch, the complete series of gills, the mem- 

 branous roof of the brain, and the fact that the young emerge from 

 the egg like the adult. 



(2) Dipneumona, including Lepidosteus and Protopterus, charac- 

 terised by the bilobed air-bladder, the vestigial paired fins devoid 

 of fin-rays and resembling filaments the true gill on the hyoid arch 

 and the reduction of the other gills, the nervous roof to the brain, 

 the fact that the larvae have external gills and the eel-like form of 

 the adult. 



Before dismissing the Pisces and passing on to consider land 

 animals, we may endeavour to sum up the general conclusions to 

 which our study has led us. 



The most primitive fish must have resembled in many points 

 the Chondrichthyes, as for example in possessing no bone and in 

 having as sole skeleton the placoid denticles. The Chondrichthyes 

 have been enabled to survive to the present day chiefly by the 

 development of the oviduct into a womb and the great advantage 

 which this gives in enabling them to launch their young fully equipped 

 for the battle of life instead of turning them loose as helpless larvae 

 as do almost all Osteichthyes. The air-bladder of Osteichthyes 

 corresponds to the lungs of land animals and from observations on 

 the development of lungs we are led to regard these as a posterior 



