The occipital segment is completely ossified, and an interorbital septum or bony partition separating the two orbits is usually developed to a greater or less extent. The skull generally articulates with the vertebral column by a single occipital condyle into the composition of which the exoccipitals and basi-occipital enter in varying proportions. The pro-otic ossifies, and either remains distinct from the epi-otic[65] and opisthotic throughout life, or unites with them only after they have fused with the adjacent bones. The hyoid and branchial arches are much reduced; and the representative of the hyomandibular is connected with the auditory apparatus, forming the auditory ossicles[66]. Each ramus of the mandible always consists of a cartilage bone, the articular, and several membrane bones. The mandible articulates with the cranium by means of a quadrate. The ribs in Birds and some Reptiles bear _uncinate processes_, i.e. small, flat, bony or cartilaginous plates projecting backwards from their posterior borders. The sternum is not transversely segmented as in mammals, and there are commonly distinct cervical ribs. The ankle joint is intertarsal, or situated between the proximal and distal row of tarsal bones, not cruro-tarsal as in Mammalia. CLASS I. REPTILIA[67]. The axial skeleton is generally long, and that of the limbs frequently comparatively short, or sometimes absent. The exoskeleton generally has the form of epidermal scales, which are often combined with underlying bony dermal plates or scutes and may sometimes form a continuous armour. Neither feathers nor true hairs are ever present. The vertebral column is generally divisible into the five usual regions. The centra of the vertebrae vary enormously, and may be amphicoelous, procoelous, opisthocoelous or flat, but they never have saddle-shaped articulating surfaces. The quadrate is always large, and is sometimes fixed, sometimes movable. A transpalatine bone uniting the pterygoid and maxillae is generally present. Free ribs are often borne along almost the whole length of the trunk and tail, and often occur attached to the cervical vertebrae. The sacrum is generally composed of two vertebrae which are united with the ilia by means of expanded ribs. The sternum is rhomboidal, and may either be cartilaginous or formed of cartilage bone, but never of membrane bone; it differs from that of birds also in the fact that it does not ossify from two or more centres. An interclavicle is generally present. There are always more than three digits in the manus, and never less than three in the pes. In all living reptiles the ilia are prolonged further behind the acetabula than in front of them, and the bones of the pelvis remain as a rule, distinct from one another throughout life. The pubes (pre-pubes) and ischia both commonly meet in ventral symphysis, and the acetabula are wholly or almost wholly ossified. The metatarsals are not ankylosed together. _Order 1._ THEROMORPHA[68].