504 DIPNOI [CH. 



basals, as in Elasmobranchia. Of these the two outer are ossified, 

 while the central one remains cartilage. 



Calamoichthys is an eel-like form, but agrees in all essentials 

 with Polypterus. With the Polypterini were formerly associated 

 a large number of fossil fish common in the Devonian and Car- 

 boniferous strata, which Goodrich has separated under the name 

 OSTEOLEPIDOTI. These agree with Polypterini in having a diphy- 

 cercal tail and a scaly lobe in the pectoral fin, and in having 

 shining rhomboid scales. It appears, however, that the pectoral 

 fin is biseriate, of the type to be described in the next order, and 

 according to Goodrich the scales which he terms cosmoid, have 

 a different structure from the ganoid scales. In cosmoid scales, 

 under a very thin layer of shining substance, there is a row of 

 pulp-cavities from which radiate dentinal canals, suggesting that 

 this layer of the scales is derived from the fusion of placoid 

 denticles. 



Order VI. Dipnoi. 



The Dipnoi, or Lung-fish, including the genera Protopterus 

 from the swamps of Africa, Lepidosiren from the swamps of South 

 America, and C&ratodus from the Australian rivers, are distinguished 



FIG. 246. Lepidosiren parafloxa. Male, showing the feathered pelvic fins of 

 the breeding season. Much reduced. From Graham Kerr. 



for their power of breathing air as well as water. In the case of 

 Lepidosiren and Protopterus this is necessary, because the swamps 

 in which they live dry up in the dry season, a time which the 

 fish pass through buried in the mud, enclosed in air-containing 

 chambers or cocoons which communicate with the surface by 

 breathing-pores. . Ceratodus inhabits rivers in which the water turns 

 foetid at certain seasons of the year from decaying vegetation, and 

 during this period it breathes air. The air-bladder is bilobed in 

 Protopterus and Lepidosiren, undivided in Ceratodus ; its walls are 

 developed into pockets or sacculi, like those of the lungs of land 

 animals, and the two lobes unite to form a duct which passes 



