. The chordates. Chordata. Class Mammalia 563 Especially characteristic are peculiarities of the mammalian jaw and of the closely adjacent bony mechanism of the ear. In modern reptiles the joint between lower and upper jaw is between the articu- lar, a bone formed by ossification of the posterior end of I he embryonic cartilaginous lower jaw, and the quadrate, a cartilage bone of the upper jaw. The squamosal, a dermal bone, adjoins the quadrate but has no part in the joint (Fig. 436/1). In adult mammals the lower jaw is composed entirely of dermal bone and articulates with a bone which unmista
. The chordates. Chordata. Class Mammalia 563 Especially characteristic are peculiarities of the mammalian jaw and of the closely adjacent bony mechanism of the ear. In modern reptiles the joint between lower and upper jaw is between the articu- lar, a bone formed by ossification of the posterior end of I he embryonic cartilaginous lower jaw, and the quadrate, a cartilage bone of the upper jaw. The squamosal, a dermal bone, adjoins the quadrate but has no part in the joint (Fig. 436/1). In adult mammals the lower jaw is composed entirely of dermal bone and articulates with a bone which unmistakably corresponds to the reptilian squamosal (Fig. 436Z?). The definitive upper jaw contains no bone recognizable as a quadrate, and there is no articular in the lower jaw. In the reptilian ear the tympanic membrane is connected with the bony wall of the auditory capsule by a single skeletal structure (partly cartilaginous, partly bony), the columella. In mammals the connection between tympanic membrane and auditory capsule is by means of a jointed chain of three ossicles, the malleus, incus, and stapes (see Fig. 518). Mammalian teeth exhibit a highly characteristic differentiation (Fig. 4365). In vertebrates other than mammals, with rare exceptions, the teeth are of simple form and, in a given species of animal, they are of the same form on all regions of the jaws—the condition known as homodont. But in most mammals there is local differentiation of teeth—a heterodont condition. At the front of the jaws are sharp- edged cutting teeth (incisors) in each jaw. At either end of the row of incisors there is a characteristic conical or pointed tooth (canine), best developed in carnivores. Behind each canine is a series of "'cheek- teeth," premolars and molars, usually of heavier build and variously. Fig. 136. (A) Skull of turtle, Emys. (B) Skull of dog. (BP) Cartilage; (C. occ.) occipital condyle; (F) frontal; (Fl) postfrontal; (Finf) infraorbital foramen; (HK) horny s
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