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.
16. Proreptilia are represented by the Permian genera Eryops and Cricotus. Until quite recently these and many other fossils from the Carboniferous strata were looked upon as Amphibia, while many undoubted fossil Amphibia were mistaken for reptiles, as indicated by the frequent termination '-saurus' in their names.
The nearest living representative of these extinct Proreptilia is the New Zealand reptile Hatteria, or Sphenodon, close relations of which are known from the Upper Trias; while others—e.g., Palæohatteria—have been discovered in the Permian. Anyhow, Sphenodon is the reptile which stands nearest to the main stem of our ancestry.
The most important characteristics of the Reptilia, which mark a higher stage or level, are (1) The entire suppression of the gills—although during the embryonic development the gill-clefts still appear in all reptiles, birds, and mammals; (2) The development of an amnion and an allantois, both for the embryonic life only, but so characteristic that all these animals are comprised under the name of Amniota; (3) The articulation of the skull with the first neck vertebræ by well-developed condyles, either single (really triple) or double (such a condylar arrangement begins with the Amphibia, but only the two lateral condyles are developed, while the middle portion, belonging to the basi-occipital element, remains rudimentary[22]); (4) The formation of centra, or bodies of the vertebræ, mainly by a ventral pair of the original quadruple constituents, or arcualia.
17. Between the Proreptilia and the Mammalia, which latter occur in the Upper Triassic epoch, we have necessarily to intercalate a group of very low reptiles, which are still so generalized that their descendants could branch off either into the Reptilia proper or into the Mammalia. The changes concerned chiefly the brain and the heart; of the skeleton, the skull and the pelvis; and, of the tegumentary structures, the formation of a hairy covering. Many such creatures existed in the Triassic epoch—namely, the Theromorpha—some of which indeed possess so many characteristics which otherwise occur in the Mammalia only, that these creatures have been termed Sauro-Mammalia. However, it has to be emphasized that none of the Theromorpha hitherto discovered fulfils all the requirements which would entitle them to this important linking position. They only give us an approximate idea of what this link was like.
18. Stage of the Promammalia, or Prototheria. The only surviving members are the famous duck-bill, Ornithorhynchus, and the spiny ant-eaters, Echidna and Proechidna, of the Australian region. These few genera, however, differ so much from one another in various important respects that they cannot but be remnants of an originally much larger group. Indeed, many fossils from the Upper Triassic and from the Jurassic strata have without much doubt to be referred to the Prototheria. The Prototheria are typical mammals, because they possess the following characteristics: The heart is completely quadrilocular; the blood is warm, and its red corpuscles have, owing to the loss of their nucleus, been modified from biconvex into biconcave discs; they have a hairy coat and sweat glands, and two occipital condyles; the ilio-sacral connection is preacetabular; the ankle-joint is cruro-tarsal; the quadrate bone of the Reptilia has ceased to carry the under jaw, which now articulates directly with the squamosal portion of the skull. Their low position is shown by the retention of the following reptilian features: Complete coracoid bones and a T-shaped interclavicle; a cloaca, or common chamber for the passage of the fæces, the genital and the urinary products; they are still oviparous; the embryo develops without a chorion, and is therefore not nourished through a placenta. Even the milk glands, which are absolutely peculiar to the Mammalia, are still in a very primitive stage, and do not yet produce milk proper; and there is only a temporary shallow marsupium.
19. Stage of Metatheria, or Marsupialia, are direct descendants of Prototheria; but they show higher development by the reduction of the coracoid bones and the interclavicle. The original cloaca is divided into a rectal chamber and a uro-genital sinus, completely separated, at least in the males; they are viviparous; the young are received into a permanent marsupium, in the walls of which are formed typical milk glands and nipples, but the embryo is still devoid of a placenta, although some recent marsupials show indications of such an organ. The corpus callosum in the brain is still very weak.