DIPNOI. 97 



of a triple division. Along the middle of the fin runs a jointed |_Case 28. 

 axis, the joints gradually becoming smaller and thinner towards the 

 extremity ; each joint bears on each side a three-, two-, or one- 

 jointed branch. This "axial" arrangement, which evidently 

 represents one of the first and lowest conditions of the skeleton of 

 the limb of Vertebrates, is found in Ceratodus with the branches, 

 but in Lepidosiren the jointed axis only has been preserved, with 

 the addition of rudimentary rays in Protopterus. 



Three recent genera and four species are known, viz.:— Lepidosiren 

 paradoxa (fig. 88), from the system of the River Amazons, and so 



Fig. 88. 



Lepidosiren paradoxa. (From the River Amazon.) 



rare that no specimen could be procured hitherto for the British 

 Museum ; Protopterus annectens, spread over the whole of tropical 

 Africa and common ; and Ceratodus, from the fresh waters of 

 Queensland, with two species, C.forsteri and C. miolepis. Proto- 

 pterus lives in shallow waters which periodically dry up. During 

 the'dry season they form a cavity in the mud, the inside of which 

 is lined with a capsule of mucus, and from which they emerge again 

 when the rains refill the pools inhabited by them. The balls of 

 clay] containing the fishes in a torpid condition are sometimes 



