Page:Popular Science Monthly Volume 11.djvu/21

 Rh of the ventral surface. With two African Ganoids, Polypterus and Calamoichthys (as also stated by Müller, and verified by the writer as to the latter genus) the sac is double, and communicates with the ventral side in the median line; but it is slightly cellular, as in Menobranchus.



Finally, in the "mudfishes" of Africa and South America (Protopterus and Lepidosiren) the duct is ventral, and the air-bladder is a double and lung-like sac with stiff walls.

This series seems to connect the air-bladder of the fishes with the lungs of the true aërial vertebrates, and to remove the objection