In the silent depths of the large lakes of North America there is a fish called the Bony pike,[15] a huge fellow often six feet long, with a long beak-shaped mouth, which he snaps as he goes, devouring everything that comes in his way. This fish has his body covered with lozenge-shaped, bony, enamelled scales, like the fish of long ago, and so too has the strange Bichir,[16] which wanders above the cataracts of the Nile, with its row of eight to eighteen fins raised upon its back like tiny sails. Then again there are the curious calf-fish of North America,[17] of the Amazons,[18] of the Nile,[19] and of the rivers of Queensland in Australia.[20] These all have gristly skeletons, and together with the sharks and sturgeons make up all that remains of those strange shadows of the past moving among the bony fishes of to-day. The mud-fishes are indeed the most curious of all, for they breathe both water and air, and in the Nile and Gambia often coil themselves round in the mud when the water goes down, and, lining their bed with slime, sleep comfortably till the rains refill the pools with water.[21] The fact is they have two quite separate ways of breathing. They have gills with which they can take air out of the water like other fish, and these they always use when they can. But they have a tube in their throat leading into the air-bladder lying under their backbone, and through this they can breathe in air when they cannot get it from the water. In _Amia_ especially, which is a true enamel-scaled fish, this air-bladder is divided into numerous cells, and it breathes with it just as with a lung. [Illustration: Fig. 8. The Ceratodus of Queensland, an air-breathing and water-breathing mud-fish of the ancient type, with paddle-fins.]