13. Closely connected with the Elasmobranchs in a wider sense are the _Crossopterygii_, which begin in the Devonian age as a large group, but have left only two survivals, the African Polypterus and Calamoichthys. They are possessed of dermal bones and other ossifications, and are characterized by 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 are characterized as follows: The paired fins still retain the archipterygial form (namely, one axis with biserial rays); the heart is already trilocular, and receives blood which is mixed arterial and venous, owing to the gills being retained, while the air-bladder has been modified into a lung. In fact, the generalized Dipnoi form the actual link between fishes and _Amphibia_. 15. _Amphibia._ The earliest amphibian fossils occur in the Carboniferous strata. They alone--the Stegocephali or Phractamphibia--stand in the ancestral line, while the Lissamphibia, to which all the recent forms belong, are side-branches. The Stegocephali are the earliest Tetrapoda, the archipterygial paired fins having been transformed into the pentadactyle fore and hind limbs, which are so characteristic of all the higher Vertebrata. The cranium is roofed over by dermal bones, of which, besides others, supra-occipitals, supra-orbitals, and supra-temporals are always present. The lowest members (Branchiosauri) still retained gills besides the lungs, while others (Microsauri) have lost the gills. Be it remembered that all the recent Amphibia still undergo the same metamorphosis during their ontogenetic development. In the very important Temnospondyli, a subgroup of the Stegocephali--_e.g._, Trimerorhachis of the Lower Red Sandstone or Lower Permian--the component cartilaginous or bony units which compose the vertebræ still remained in a separate, unfused state, showing at the same time an arrangement whence has arisen that which is typical of the Amniota. The same applies to the limbs and their girdles. In fact, the Stegocephali, taken as a whole, lead imperceptibly to the _Proreptilia_.