Reptiles

Classification.

The classification of reptiles is necessarily based on skeletal characters, and is still in a state of flux. The existing divergences of view are not very important; they relate to the phylogenetic position of a few orders, and do not seriously affect the main outline.

Class Reptilia.

Tetrapodous vertebrates, which breathe air throughout their life. The body temperature is variable. The heart possesses a sinus venosus, two auricles and a ventricle in completely or completely divided into two; there is no conus arteriosus. Both systemic arches persist. The red blood corpuscles are nucleated, oval and biconvex. The kidney is a metanephros, and there is an allantoic bladder, in most forms. There is a cloaca, which in living reptiles is divided into a series of regions. The skin is either naked or covered with scales, never with feathers or hair. It includes very few glands, always placed in special situa tions, and not generally distributed. The skeleton is ossified. The skull comprises a cranium, of cartilage bones, and an extensive series of bones, which, dermal in origin, sink in and become mem brane bones in the later forms. The occipital condyle is single or double. The lower jaw articulates with a quadrate bone and is built up of a number of bones. There is a rod-like columella auris. The vertebrae consist mainly of centra and neural arches, inter centra, when present, being small. Ribs occur on all precaudal vertebrae, those in the thoracic region joining to form a sternum in the mid-ventral line.

The pectoral girdle, when fully developed, includes at least a scapula and precoracoid, clavicles and an interclavicle. The pelvic girdle, except in one or two cases, articulates with two or more sacral ribs. The limbs are primitively pentadactyle and the phalan geal formula 2, 3, 4, 5, 3 or 4.

Fertilization is internal, the eggs are large and yolk laden, usu ally laid, when they are surrounded by a coat of albumen and a shell which is often calcified. Cleavage is meroblastic, a primitive streak is not formed, the embryo is surrounded by an amnion and an allantois is developed.

Super-order. Cotylosauria. Archaic reptiles in which the temporal region of the skull is completely covered by a continu ous roof of dermal bones. Stapes either articulating with the quadrate or ending in a tympanic notch. Lower jaw usually with more than one coronoid. Presacral vertebrae (except in Pantylus) with very heavy neural arches with horizontal zygapophysial ar ticular faces. Ribs one or two headed. No ossified sternum. Ab

dominal ribs sometimes present as fine bony rods, Shoulder girdle with scapula and precoracoid at least, a coracoid usually present in addition. Cleithra usually, clavicles and an interclavicle always present. Humerus with (usually) a screw-shaped head, short and with widely expanded extremities. Pelvis, plate-like, the suture between the pubis and ischium extending from the acetabulum to the middle line.

Order 1. Seymouriamorpha. Cotylosaurs in which the skull greatly resembles in all external features that of the Embolomerous Labyrinthodonts, the dermal bones are sculptured and the otic notch extends far forward below the tabular and supratemporal, so that the quadrate slopes backward. Stapes ending in the otic notch. Fenestra ovalis low down on the cranium, below the level of the base of the brain. Intercentra present and very large, ribs one or two headed. Only one sacral vertebra. Shoulder girdle without coracoid or cleithrum, limbs primitive.

Upper Carboniferous to Upper Permian. Families, Seymou riidae,Kotlassiidae.

Order 2. Captorkinomorpha. Cotylosaurs in which the otic notch has been obliterated by a movement backward of the upper end of the quadrate. Dermosupraoccipitals and tabulars, when present, restricted to the occipital surface. Stapes articulating distally with the quadrate. Brain case behind the incisura pro oticum short and high, fenestra ovalis ventrally situated. Intercen tra usually present. One or two sacral vertebrae. Shoulder girdle with both precoracoid and coracoid. Cleithrum present or absent. Limbs primitive, though sometimes slender.

Lower Permian. Families Captorhinidae, Pantylidae, Limnos celidae.

Order 3. Diadectomorpha. Cotylosaurs in which the otic notch is enlarged by a movement forward of the lower end of the quad rate, dermosupra occipitals and tabulars when present on the upper surface of the skull, the latter overhanging the otic notch. Stapes terminating freely in the otic notch. Brain case long, fenestra ovalis placed at about the middle of its height. Inter centra usually present. Two to four sacral vertebrae. Shoulder girdle with a scapula alone or with three cartilage bones. Cleith rum usually present. Humerus always with expanded ends but often of advanced structure. Ilium sometimes backwardly directed.