Seymouria is a small animal about 2 ft. in length, with a com paratively small head, no visible neck, a somewhat stumpy body, and a short tail. The limbs were very muscular, but short. The hands and feet were placed far away from the middle line, and the stride was exceedingly small Each limb has five digits.
The skull of Seymouria consists of two parts, which could have easily been separated from one another. These are the brain-case, made of bones which have replaced the cartilage which existed in the embryonic skull, and a superficial coating covering the whole outer surface of the head (except for the nostrils, orbits and pineal foramen), and the roof of the mouth, made of bones which have developed in the lower layers of the skin. The pat tern formed by these dermal bones is identical with that which is found in the more primitive Labyrinthodont Amphibia, and is important, because from it the structure of the corresponding parts of the skulls of all other reptiles can be derived, by a process of reduction. The palate of Seymouria is, in essence, identical with that of an Embolomerous Labyrinthodont. The brain case, however, differs somewhat from those of the Amphibia. For example, the single occipital condyle is convex instead of being concave, and there is a large fenestra ovalis leading into the ear which does not exist in the Embolomeri. There are also variations in other details of the structure of the otic region. In the lower jaw, Seymouria is identical with an amphibian, but the vertebral column is very different.
In the amphibian the first vertebra articulates with the con dyle by a disc-shaped inter-centrum followed by a disc-shaped centrum, of the same character as those which succeed it. In Seymouria the rounded condyle articulates below with a con cavity on an inter-centrum which represents only the lower half of that of the amphibian, and with facets carried on the lower ends of the two halves of the neural arch. The centrum of the atlas is a curious trefoil-shaped bone which fits in between the three elements which articulate with the condyle ; this arrange ment is completely reptilian. The structure of a vertebra from the middle of the back of Seymouria is quite peculiar. There is a small cylindrical centrum separated from the next by an inter centrum having the shape of half a disc. The neural arch is enormously heavy, it articulates with the centrum alone and the pre- and post-zygopotheses are produced laterally as masses of bone which overhang the much smaller centrum. The ar ticulating faces are quite flat and placed horizontally, so that the back, although free to move from side to side, must have been extremely stiff dorso-ventrally.
Vertebrae of this type are known in no amphibian, but in a less exaggerated form occur in many of the more primitive rep tiles. It is reasonable to believe that they were evolved as a clumsy method of giving that stiffness to the back which is necessary to an animal which, living in air, has to support the whole of its weight. The ribs of Seymouria do not differ essentially from those of some Labyrinthodonts. The limb girdles and limbs are of the amphibian pattern except in one or two de tails, e.g., the occurrence of an ent-epi-condylar foramen piercing the humerus, and the number of the phalanges, which is two, three, four, five, three, the characteristic reptilian number.
Thus it is possible to be in doubt whether an extinct animal whose skeleton is completely known is an amphibian or a reptile, the break between the two being completely bridged so far as the skeleton is concerned. From a skeleton similar to that of Sey mouria it is possible to derive those of all later reptiles, and in this way, by sorting out separate evolutionary lines to establish a classification which may express not only differences of structure existing between the animals contained in it, but something of their phylogenetic relationships.
Evolutionary Development.—The reptiles, as a whole, with a few doubtful exceptions, divide into two great branches, the mammal-like reptiles and the rest. The differences between the members of these two groups are to be found mainly in the structure of the brain case and the back of the skull. In all the mammal-like reptiles the inner ear lies in the lower part of the side wall of the brain-case, the brain extending far above it, whilst in all other reptiles the ear extends throughout the whole of the side wall of the cranium and is not exceeded in height by the brain. In Seymouria the tympanic membrane is stretched across a notch on the outer surface at the back of the skull; in the mammal-like reptiles this notch is destroyed, so that the occipital surface of the skull is flat and the tympanic membrane, if it exists at all, lies ventrally in the neighbourhood of the hinder end of the lower jaw, to one of whose elements it is attached. In the remaining reptiles the tympanic or otic notch is preserved, bounded above by a special process of the squamosal or tabular bone, and by the free distal extremity of the paroccipital. The tympanic membrane, when present, lies high up on the side of the head, far removed from the lower jaw. In the mammal-like rep tiles the stapes is attached directly to the quadrate bone, whilst in the others it is continued by an extra columella which is inserted into the tympanic membrane.