TEETH. SKULL. 629 



lower jaw. Of the former the anterior pair does not appear 

 above the gum and is absorbed before birth, while the pos- 

 terior is shed in the third week after birth ; the milk incisors 

 of the lower jaw are absorbed before birth. The large incisors 

 appear always to be rootless and to grow throughout life. 

 They extend far back into the jaws and are much curved. 

 They have enamel only on their anterior faces except in the 

 Duplicidentata, in which it extends on to their inner sides. 

 The small incisors of the latter are also rootless and have 

 enamel on both faces. By continuous wear they are kept to 

 a sharp edge of enamel. In many rodents the enamel of 

 the incisors is stained a yellow colour. The milk molars have 

 roots, and are brachyodont. The permanent grinders are 

 either brachyodont and rooted, or hypsodont and rootless, 

 growing throughout life. In the latter case they are curved 

 as in Toxodon, so as to take the pressure off the growing 

 pulp. There is considerable variety in the surface of the 

 crown according to the food (vide Tullberg, op. cit.). In 

 omnivorous forms they are brachyodont and bunodont, the 

 enamel of the crown not being much folded and wearing 

 through with use, so that the surface comes to consist of 

 dentine surrounded by an enamel ring. In the herbivorous 

 forms, however, in which the wear is greater, they are hypso- 

 dont and continue to grow throughout life or through part 

 of life, and the enamel is deeply folded into transverse ridges, 

 the valleys between which may be filled in with cement. As 

 a rule there are two or three ridges, but in the capybara, the 

 last of the four grinders has a great number of these transverse 

 enamel folds and appears to consist of many laminae embedded 

 in cement. In these cases of lophodont molars the unworn 

 tooth is tuberculated, the laminated pattern becoming 

 apparent when the tubercles are worn off. The enamel 

 varies in the arrangement of the prisms in different families, 

 and in some cases dentinal tubes extend into it. 



The long axis of the articular condyle of the lower jaw is 

 directed longitudinally and the squamosal has no postglenoid 

 process. There is a large tympanic bulla, and the tympanic is 

 generally prolonged into a tubular meatus : it often remains 

 distinct from the squamosal and periotic. The paroccipital 

 processes of the exoccipital are long. The jugal occupies the 



