The conformity of pattern in the dermal, semiderinal, or neurodermal bones of the outwardly well-ossified skull of Polyptents, Lepidosteus, Sturio, and other salainandroid-ganoid fishes, with well-developed lung-like air-bladders, and of the same skull-bones in the Archegosaurus and the Labyrintho donts ; the persistence of the notochord (chorda dorsalis) in Archegosaurus, as in Sturio ; the persistence of the notochord and branchial arches in Archegosaurus, as in Lepidosiren ; the absence of occipital condyle or condyles in Archegosaurus, as in Lepidosiren ; the presence of labyrinthic teeth in A rehey o saurus, as in Lepidosteus and Labyrinthodon ; the large median and lateral throat-plates in Archegosaurus, as in Megalichthys, and in the modern Arapaima and Lepidosteus ; —all these characters point to one great natural group, peculiar for the of development, and blending together fishes and reptiles within the limits of such group. The salamandroid (or so-called "sauroid") Ganoids--Lepi dostetts and Polypterus--are the most ichthyoid, the true Labyrinthodonts are the most sauroid, of the group. The Lepidosiren and Archegosaurus are intermediate gradations, one having more of the piscine, the other more of the reptilian characters. The Archegosaurus conducts the march of develop ment from the fish proper to the labyrinthodont type ; the Lepidosiren conducts it to the perennibranchiate batrachian type. Both illustrate the artificiality of the supposed class distinction between fishes and reptiles, and the naturality of the " Ilfematocrya," or cold-blooded Vertebrata, as the one unbroken progressive series. There is nothing in the known structure of the so-named Archegosaurus or Afastod,onsaurus that truly indicates a belonging to the saurian or crocodilian order of reptiles. The exterior ossifications of the skull and the canine-shaped labyrinthic teeth are both examples of the salamandroid modification of the ganoid type of fishes.
The small proportion of the fore limb of the Mystriosaurus in nowise illustrates this alleged saurian affinity ; for though it be as short as in Archegosaurus, it is as perfectly constructed as in the crocodile, whereas the short fore limb of Archego saurus is constructed after the simple type of that of the Proteus and Siren. But the futility of this argument of the sauroid affinities is made manifest by the proportions of the hind limb of Archegosaurus. As in Proteus and Amphiunta, it is as stunted as the fore limb ; whereas in Mystriosaurus, as in other Teleosaurians, the hind limbs are relatively larger and stronger than in the existing crocodiles.
The bones of the head still include the supplementary "post orbitals" and " supra-teinporals," but there are small temporal and other vacuities between the cranial bones : a " foramen parietale," a single convex occipital condyle, and one vomer which is edentulous. Two antorbital nostrils. Vertebral centra, ossified, biconcave. Pleur apophyses of the trunk long and bent, the anterior ones with bifurcate heads. Teeth with converging folds of
cement at their base ; implanted in a common alveolar groove, and confined to the premaxillary, and premandibular bones. Premaxillaries much exceeding the maxillaries in size. Orbit very large ; a circle of sclerotic plates. Limbs natatory ; with more than five multi-articulate digits ; no sacrum. With the retention of characters which indicate, as in the preceding orders, an affinity to the higher Ganoid fishes, the present ex clusively marine Reptilia more directly exemplify the ichthyic type in the proportions of the premaxillary and maxillary bones, in the shortness and great number of the biconcave vertebra, in the length of the pleur apophyses of the vertebra near the head, in the large proportional size of the eyeball and its well-ossified sclerotic coat, and especially in the structure of the pectoral and ventral fins.
It has been usual to unite the present with the following order in the same group, called Enaliosauria or sea-lizards. They were adapted for marine life, but breathed the air like the Cetacea : they were, however, " cold-blooded," or of a low temperature, like crocodiles and other reptiles. The proof that the Enaliosaurs respired, atmospheric air immediately, and did not breathe water by means of gills like fishes, is afforded by the absence of the bony framework of the gill apparatus, and by the presence, position, and structure of the air-passages leading from the nostrils to the mouth, and also by the bony mechanism of the capacious chest or thoracic abdominal cavity ; all of which characters have been demon strated by their fossil skeletons. With these characters the sea-lizards combined the presence of two pairs of limbs shaped like fins, and adapted for swimming.
The group of reptiles so termed includes all those which have any part of the thoracic-abdominal cavity encompassed by moveable ribs. The first character distinguishes them from the Batrachia and Chelonia with fin-shaped limbs.
The Enaliosauria, however, do not form a strictly natural group ; for this is based upon a single character relating to merely the medium of life and locomotion. Some of the labyrinthodont reptiles may have had their limbs in structure and shape as paddles ; but more important modifications of structure would keep them apart, like the lower Batrachia and the Chelonia, from the more lizard-like reptiles called Enaliosauria.
In this group there are two divisions,---one characterized by having five digits in the fin, the other by having more than that typical number. The pentadactyle division may be sub-divided into those in which the ilio-pubic arch is attached to a sacrum and those on which it is freely sus pended or not so attached. The polydactyle division presents a general type of structure more conformable with that of which the Archegosaurs and Labyrinthodonts manifest two phases of development, and in which the ascent from the gano-salamandroid fishes reaches its culminating point in Ichthyosaurus.