There is an obturator foramen in the pelvis. The limbs are short and powerful, the track wide, and the digital formula 2, 3, 3, 3, 3.
Upper Permian to Middle Trias. Division into families not yet carried out.
Order 5. Theriodontia. Theromorpha in which there is a differen tiation of the dentition into incisors, canine and cheek teeth. The face is usually long, the temporal fossa, short in primitive forms, elongated in the more advanced types, the parietal entering into its border.
Quadrate and quadrato-jugal, fused, much reduced and carried in a recess on the front face of the squamosal.
Pterygoids forming great transverse flanges, behind which they suddenly contract to form a narrow girder extending back to the basisphenoid. Palate at first with the large posterior nares placed anteriorly, becoming vaulted, the air passage being finally cut off from that for the food by a secondary palate. The dentary, always extending above the surangular, in a free coronoid process. Limbs and their girdles variable.
Sub-order I. Gorgonopsia. Primitive Theriodonts, with the postorbital and squamosal meeting above the temporal fossa. Single occipital condyle : No sub-orbital vacuities. No secondary palate. Scapula without acromion, plate-like pelvis, digital formula (of hand) 2, 3, 4, 5, 3 Upper Permian.
Sub-order 2. Cynodontia. Advanced Theriodonts, with the parietal entering the temporal fossa. No sub-orbital vacuities. A secondary palate. Pair of exoccipital condyles. Scapula with acromion. Pelvis with an obturator foramen. Limbs modernized, digital formula 2, 3, 3, 3, 3.
Top of the Permian and Lower Trias.
Sub-order 3. Therocephalia. Primitive Theriodonts with large temporal vacuities into whose border the parietal always enters. Large sub-orbital vacuities. No secondary palate or vaulting of the mid line of the anterior part of the palate. Single occipital condyle.
Upper Permian.
Sub-order 4. Bauriamorpha. Advanced Theriodonts, with the parietal forming part of the temporal fossa. Large sub-orbital vacuities, a secondary palate. Single occipital condyle.
Lower to Upper Trias.
Order 6. Thalattosauria. A group of marine reptiles, still incom pletely known, but perhaps allied to the Pelycosauria. If so inter preted they may be defined by the following characters :—Skull with a very elongated face formed by the maxillae and premaxillae, nostrils dorsal and immediately in front of the large orbit, nasals small. The large temporal fossa is entirely lateral and is bounded above by the postorbital and squamosal. Quadrate large. A supra temporal present. Parietals short and wide. Vertebrae with bicon cave centra which are short cylinders, ribs single-headed. Scapula and coracoid incompletely ossified. Humerus with expanded ends and a twisted shaft. Radius and ulna short flattened bones.
Upper Trias.
Super-order Archosauria (Diaptosauria). Reptiles in which the temporal region of the skull is perforated by two vacuities, the upper of these, the supratemporal fossa is bounded by the parietal, supratemporal, squamosal and postorbital the lower, the infra temporal fossa lies between the postorbital squamosal, quadrato jugal and jugal bones.
The brain, at any rate in the later forms, is completely enclosed by bone, a pair of latero-sphenoids surrounding the cerebral hemi spheres and stretching back to have a suture with the pro-otic. The epipterygoid forms no part of the wall of the cranial cavity. The fenestra ovalis lies half way up the wall of the brain case. There is always a distinct neck, often of eight vertebrae.
The pectoral girdle contains a scapula and precoracoid on each side, the true coracoid never appearing. Cleithra are never present. The sternum usually ossifies from a pair of centres. The limbs are never of the primitive Cotylosaurin character, and are often very highly modified. The digital formula is 2, 3, 4, 5, 4 or 3.
Order 1. Thecodontia. Primitive Archosauria in which a supra temporal, tabular and interparietal may be present in the skull. A preorbital vacuity may be present or absent. The ribs may have one or two heads and a sternum if ossified is paired.