Page:The Last link.djvu/71

Rh their lobate paired fins, which have a thick axis beset with biserial fin rays. Their gill-clefts are covered by an operculum, and they have a well-developed air-bladder. Whilst they are in many respects more highly developed than the Elasmobranchs, and are intimately connected with the typical Ganoids and other bony fishes (all of which form a great, manifold side-branch of the general vertebrate stem), they stand in many other respects (notably, the structure of the paired fins, the vertebral column, and the air-bladder) nearer the main-stem of our own ancestral line.

14. This is shown by their intimate relation to the Dipnoi, which are still represented by the Australian, African, and South American mud-fishes: Ceratodus, Protopterus, and Lepidosiren. The genus Ceratodus existed in the Upper Trias, whence various other unmistakably dipnoous forms lead down through the Carboniferous (e.g., Ctenodus) to the Devonian strata—e.g., Dipterus. They