X GANOIDS, LUNG-FISH 373 



ductive cells, on which the phenomena of inheritance depend, has been 

 made by Agar upon Lepidosiren. 



The group Dipnoi, represented during ancient geological periods by 

 probably numerous genera and species, distributed widely over the 

 earth's surface, is to-day on the verge of extinction, being represented 

 only by the three surviving genera mentioned above. These afford 

 an excellent example of the discontinuous geographical distribution 

 already alluded to as often characteristic of the present-day representa- 

 tives of ancient groups of animals, for they inhabit respectively the 

 swamps of tropical South America (Lepidosiren), and of tropical Africa 

 (Protopterus), and the Burnet and Mary Rivers of Queensland (Ceratodus). 

 The first of these to become known to science was Lepidosiren, two 

 specimens of which were brought from Brazil in 1837. The creature 

 remained a great rarity until the 'nineties of last century when its scientific 

 investigation in its native swamps was seriously taken up. Protopterus 

 became known very soon after Lepidosiren — a preserved specimen in the 

 College of Surgeons' museum in London, brought from the River Gambia, 

 attracting the attention of Owen. For many years Protopterus was the 

 best known of the existing Lung-fishes owing to the fact that specimens 

 contained in the dry-season cocoon were frequently sent home to Europe 

 as curiosities. It was, however, not till 1900 that the eggs and larvae 

 were first obtained (by Budgett). The name Ceratodus was coined 

 originally for the unknown and supposedly extinct possessor of certain 

 curious fossil teeth which occur in the Rhaetic rocks of Aust Cliff near 

 Bristol. In the year 1869 similar teeth (Fig. 161, A) were observed in 

 the mouth of an undescribed fish sent from Queensland to the Sydney 

 museum, and it was realized then for the first time that Ceratodus still 

 existed alive in that remote region. The eggs and larvae of Ceratodus 

 were first discovered in 1884 by Caldwell, and their complete investiga- 

 tion, by Semon and his colleagues, followed some year^ later. 



The three surviving Lung-fishes (Fig. 160) possess somewhat cylindrical 

 bodies, stoutest in Ceratodus (A) and most slender in Lepidosiren (C), 

 tapering off gradually into the compressed tail which retains the primitive 

 symmetrical protocercal form, although in many of the extinct members 

 of the group the tail of the adult was heterocercal, no doubt in correlation 

 with their being more active and skilled swimmers. The mouth is in 

 the adult at the tip of the head. The two pairs of limbs are in Ceratodus 

 clumsy pointed paddles of a type (archipterygial) which was predominant 

 among palaeozoic fishes (Elasmobranchs, Crossopterygians, Dipnoi). In 

 the adult Protopterus and Lepidosiren the limbs have become slender and 

 degenerate. 



