The premaxillae are always large, and the orbits always communicate freely with the temporal fossae. The condyle of the mandible is elongated from before backwards, and owing to the absence of a postglenoid process to the squamosal, a backward and forward motion of the jaw can take place. The zygomatic arch is complete, but the jugal is short and only forms the middle of it. The palate is small, being sometimes, as in the hares, narrowed from before backwards, sometimes as in the mole-rats (Bathyerginae) narrowed transversely.
The thoraco-lumbar vertebrae are usually nineteen in number. Clavicles are generally present, and the acromion of the scapula is commonly very long. The feet are as a rule plantigrade, and provided with five clawed digits.
There are two main groups of Rodentia; the Duplicidentata, or Hares and Rabbits, which have two pairs of upper incisors, whose enamel extends round to the posterior surface; and the Simplicidentata, in which there is only a single pair of upper incisors, whose enamel is confined to the anterior surface. This group includes all the Rodents except the Hares and Rabbits.
Order 6. Carnivora.
The living Carnivora form a natural and well-marked group, but as is the case with so many other groups of animals, when their extinct allies are included, it becomes impossible to readily define them.
The manus and pes never have less than four well-developed digits, and these are nearly always provided with more or less pointed nails, generally with definite claws. The hallux and pollex are never opposable. The dentition is diphyodont and markedly heterodont. The teeth are always rooted, except in the case of the canines of the Walrus. The incisors are generally 3/3, and are comparatively small, while the canines are large, pointed, and slightly recurved. The cheek teeth are variable, and are generally more or less compressed and pointed; sometimes their crowns are flattened and tuberculated, but they are never divided into lobes by deep infoldings of enamel. The squamosal is drawn out into a postglenoid process, and the mandible has a large coronoid process. The condyle of the mandible is transversely elongated, and the glenoid fossa is very deep; in consequence of this arrangement the mandible can perform an up and down movement only, any rotatory or back and fore movement being impossible. The jugal is large, and the zygomatic arch generally strong, while the orbit and temporal fossa are in most cases completely confluent. The scapula has a large spine. The clavicle is never complete and is often absent, this forming an important distinction between the skeleton of a Carnivore and of any Insectivore except Potamogale. The humerus often has an ent-epicondylar foramen, and the radius and ulna, tibia and fibula are always separate. The manus is often capable of the movements of pronation and supination, and the scaphoid, lunar and centrale are in living forms always united together.
The order Carnivora includes three suborders.
Suborder (1). Creodonta[133].
This suborder contains a number of extinct Carnivora, which present very generalised characters.
The cranial cavity is very small; and the fourth upper premolar and first lower molar are not differentiated as carnassial teeth[134], as they are in modern Carnivora. The Creodonta also differ from modern Carnivora in the fact that the scaphoid and lunar are usually separate, and that the femur has a third trochanter. The feet are plantigrade.