These single-nostril animals (monorrhina) arose during the primordial period out of the skull-less animals by the anterior end of the dorsal marrow developing into the brain, and the anterior end of the dorsal skull into the skull. By the division of the single nostril of the members of the last group into two lateral halves, by the formation of a sympathetic nervous system, a jaw skeleton, a swimming bladder and two pairs of legs (breast fins or fore-legs, and ventral fins or hind-legs), arose the primæval fish (selachii), which is best represented by the still-living shark (squalacei). Out of the primæval fish arose the mud-fish (dipneusta), which is very imperfectly represented by the still-living salamander fish; the primæval fish adapting itself to land, and by the transforming of the swimming bladder into an air-breathing lung, and of the nasal cavity (which was now open into the mouth cavity) into air-passages. Their organization _might_, in some respect, be like the ceratodus and proloptems; but this is not certain. The dipneusta is an intermediate stage between the selachii and amphibia. Out of the dipneusta arose the class of amphibia, having five toes (the pentadactyla). The gill amphibians are man's most ancient ancestors of the class amphibia. Besides possessing lungs as well as the mud-fish, they retain throughout life regular gills like the still-living proteus and axolotl. Most gilled batrachia live in North America. The paddle-fins of the dipneusta changed into five-toed legs, which were afterwards transmitted to the higher vertebrata up to man. The gilled amphibia (sozobrachia) of the last group finally lost their gills but retained their tail, and tailed amphibians (sozura) were produced, such as the salamander and newt of the present day. Out of the sozura originated the primæval amniota (protamnia) by the complete loss of the gills by the formation of the amnion of the cochlea, and of the round window in the auditory organ, and of the organ of tears. Out of the protamnia originated the primary mammals (promammalia). The most closely related were the ornithostoma; they differed through having teeth in their jaws. No fossil remains of the primary mammals have as yet been found, although they lived during the trias period--they possessed a very highly developed jaw. From the primary mammal arose the pouched animals (marsupialia). Numerous representatives of this group still exist: kangaroos, pouched rats and pouched dogs. The marsupial animals developed, very probably, in the mesolithic epoch (during the Jura) out of the cloacal animals; by the division of the cloaca into the rectum and the urogenital sinus, by the formation of a nipple on the mammary gland, and the partial suppression of the clavicles. [Illustration: FIGS. I and II.--The Ceratodus Forsteri occur in the swamps of Southern Australia. Form transition between fishes and Amphibia.--_Haeckel._]