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; (F l ) postfrontal; (Finf) infraorbital foramen; (HK) 

 horny sheath on jaw; (/) olfactory foramen; (Jq, Jug) jugal; {Jrn) premaxillary; 

 (L) lacrimal; (M) maxillary; (Maud.) external auditory meatus; (Md) mandible; 

 (Mi) tympanic membrane; (N) nasal; (Na) nostril; (01) exoccipital; (O.sp) supra- 

 occipital; (P) parietal; (Pf) prefrontal; (P//) jugal (zygomatic) process of temporal; 

 (Pmx) premaxillary; (Pqa) quadratojugal; (Pi) pterygoid; (Qu) quadrate; (Si) 

 interorbital septum; (Sph) alisphenoid; (Sq, Squ) Squamosal; (Sq. ccc.) Supra- 

 occipital; (T) tympanic bulla of temporal; (Vo) vomer. (Courtesy, Wiedersheim : 

 "Grundriss der vergleichenden anatomie der Wirbeltiere," Jena, Gustav Fischer.) 



