ARCHEGOSAU'RUS, a remarkable fossil saurian reptile, so named by Gold fuss (arche gos, leader; and sauros, lizard) as constituting the real beginning of reptilian life, which had previously been considered as not extending below the pernnan series of rocks.
The skull is flattened and triangular, with rounded angles, the front one being some what lengthened. The teeth are simple cones, having a labyrinthic structure similar to that of the recent lepidosteus. The vertebral column remains in an embryonic condi tion; the arches and peripheral elements of the vertebroe are ossified; but the chorda dorsalis, which is persistent, is unprotected below. The ribs are short and almost straight, round and slender in the middle, expanded and flattened at the ends. The two pairs of limbs are nearly equal in size, and in structure very much resemble those of the proteus. They have each four long, slender digits, which obviously supported a longish, narrow-pointed paddle, adapted for swimming. Externally, the body was protected by a covering of oblong quadrangular scales, which have been preserved in Rome specimens.
Four species have been described.
The history of thti A. is shortly this: Its remains, found in the Bavarian coal-measures, had been described as those of a fish under the name of pygopterus Lucius (Agassiz). In 1844, H. von Meyer first described it limier the name of apatcon pedestris. This speci men was found in the coal-measures of Munster-Appel, in Rhenish Bavaria, and was sup posed by Meyer to be related to the salamanders, and yet not without considerable doubt; for he says, " its head might be that of a fish, as well as that of a lizard, or of a batra chitin." In 1847, Goldfuss figured and described three distinct species discovered in
large concretionary nodules of clay-ironstone, from the coal-field of Saarbrtlek, to them the generic name of A. He considered them to be a transition state between the fish-like batrachia and the lizards and crocodiles. Prof. Owen has subsequently des cribed this fossil; he makes it a remarkable connecting link between 'the reptile and the fish, and on these grounds: It is related to the salamandroid-ganoid fishes by the con formity of pattern in the plates of the external cranial skeleton, and by the persistence of the chorda (tomb's, as in the sturgeon, while it is allied to the reptiles by the persistence of the chorda dorsalis, and the branehial arches, and by the absence of the occipital con dyle, or condyles, as in lepidosiren, and by the presence of labyrinthic teeth, as in laby rinthodon, which, however, also ally it to the ganoid lepidosteus. There is thus in the A. a blending together of time characteristics of reptile and fish in one animal. It occupies a position between, and equally related to, the salamandroid-ganoid fishes on the one hand, and the labyrinthodont reptiles on the other, while the latter conduct us through the lepidosiren to the perennibranchiate batrachia.