It is customary to recognize a primitive group of reptiles, the Cotylosauria, which includes the most primitive members of each division of the reptiles. The animals included in it agree with Seymouria in that the dermal bones of the outer surface of the skull form a continuous sheet, perforated only by the nostrils, orbits and pineal foramen. This group is restricted to Permian and Triassic time, and its members thus possess very primitive limbs and limb-girdles. They are usually devoid of a neck, the shoulder-girdle lying immediately behind the head. The back is short and the vertebrae of which it is composed have very massive neural arches which articulate with one another by horizontal sur faces. The centra are perforated and transmit a continuous noto chord. There is usually a series of intercentra throughout the column. The shoulder girdle has three bony elements, the scapula, procoracoid and coracoid in each side of the cartilage girdle; all of them contribute to the glenoid cavity. Cleithra are often present, and clavicles and large inter-clavicles are universal. The fore leg is short and massive, the humerus projecting out at right angles to the animal's body and lying in a plane parallel to the ground. It can only be moved backwards and forwards, and is incapable of rotation. The elbow joint is flexible, so that the fore arm has much freedom of movement. The hand has five fingers, the number of the phalanges being 2, 3, 4, 5, 3, in the digits from I to 5. The pelvis consists of an ilium, pubis and ischium on each side, these bones meeting one another in continuous sutures, so that the whole structure is "plate-like." The hind limb pro jects out laterally and the knee was relatively inflexible; it could be stretched out straight, but, in many cases, could not be closed even to a right angle. The foot has five toes with a digital formula 2, 3, 4, 5, 4.
This super-order can be divided into three sub-groups, as fol lows :—(A.) Seymouria morpha, primitive forms represented by three genera, Solenodonsaurus, from the Upper Carboniferous of Czechoslovakia; Seymouria, from the basal Permian of Texas, and Kotlassia, from the Upper Permian of Russia, which may not properly belong to the group.
These animals possess skulls which very greatly resemble those of the Embolomerous Amphibia. (See AMPHIBIA.) These skulls have a narrow otic notch differing from that of all other reptiles ; the neural cranium is peculiar in that the powerful paroccipital processes which arise from the sides of the brain case extend out wards and upwards to support the tabular bones. The basioccipital, together with the exoccipitals, form a rounded condyle. There are well marked basisphenoidal tubera and the basipterygoid proc esses of the basisphenoid are short, and in Seymouria support the pterygoid, not directly, but through the intervention of the epi pterygoid. The parasphenoid is short and narrow. The palate is almost completely roofed with bone, there being small palatal nostrils and sub-temporal fossae in addition to a very conspicuous inter-pterygoid vacuity. The palatine bears a large tusk, the mar
ginal teeth in the upper jaw form a uniform unbroken series and exhibit an indefinite replacement. The lower jaw is built up from nine bones, dentary, splenial, post splenial, angular, sur-angular on the outer surface, the series of three coronoids between the pre articular and dentary, and an articular bone which, unlike that of all contemporary Amphibia, is not a mere part of the sur-angular. With this exception, the jaw is identical with that of a Labyrintho dont.
The vertebral column is massive, there is no distinction of neck, trunk, and lumbar region, all the vertebrae from the atlas back to the sacrum bearing two-headed ribs; there is one sacral vertebra. In the shoulder girdle a coracoid is absent, the lower part of the primary structure being ossified entirely as a pre-coracoid. The glenoid cavity has the characteristic screw-shaped form of the early Tetrapod. The humerus is an extraordinary bone, nearly as wide as it is long, whilst the fore arm is short. The hand is short and broad, the five fingers ending in small claws.
The pelvis is plate-like, the pubes and ischia being exception ally large elements ; the femur, short, broad and unusually mas sive, exactly resembles that of contemporary Amphibia. The fib ula is widened distally, and the tarsus is remarkable amongst rep tiles in possessing three bones in its proximal row, the inter medium being still separate from the tibiale. The foot is five toed, with the normal formula.
The mammal-like members of the Cotylosauria belong to the group (B.) Captorhinomorpha. This group includes a consid erable number of reptiles, all of Lower Permian age, which vary a good deal in their general structure. The most typi cal are Captorhinus and its descendant, Labidosaurus. These animals are comparatively small, with no neck, rather long bodies and not excessively long tails. They had a straddling gait, the ventral surface touching the ground and the feet being placed well away from the side of the body. The head is pointed, the face in front of the eyes narrow, whilst the temporal region was wide. The skull is completely roofed and there is no trace of an otic notch, the head having a square cut appearance posteriorly. The brain case seems to be high, and is loosely connected with the rest of the skull by the summit of the supraoccipital and the ends of the paroccipital process. The stapes is very massive and extends from the fenestra ovalis, which is placed below the level of the brain, to the quadrate to which it is attached.
The lower jaw differs from that of Seymouria by a lateral corn pression of its hinder half, and by the reduction of the coronoids to one.
The vertebral column is characterized by the massiveness of the neural arches and the obsolescence of the neural spines. The centra are small and perforated, the intercentra much reduced. All the ribs are single-headed. The remainder of the skeleton does not differ materially from that of Seymouria, which has been described above.