PERISSODACTYLA. 



595 



FiQ. Mil.— Un- 



worn upper molar 

 of horse. The an- 

 terior side of the 

 crown is to the 

 right and the 

 outer side is up- 

 wards (from 

 Flower and Ly- 

 dekker). 



i§c\pi7n I, the first premolar has no successor and falls out early (wolf 

 tooth), and the others are similar to the molars ; the last molar is not 

 more complex than the others, nor the last milk molar than the premolar 

 which succeeds it. The incisors are chisel-shaped and their crowns possess 

 a deep cavity (the mark) which is formed by a fold of the enamel and 

 eventually disappears as the tooth wears down by mastication ; the first 

 molar comes into place long before the milk-molars 

 are shed, and the canines (tusks) are small and do 

 not always appear in tlie female ; there is a con- 

 siderable diastema. The gi-inding teeth are hypso- 

 dont and grow for some time, eventually forming 

 roots. The upper molars possess four main tuber- 

 cles and two intermediate, the latter being placed 

 between the tubercles of the anterior pair and those 

 of the posterior pair. The two exterior tuber- 

 cles are semihmar in form with their concavities 

 outwards (Fig. 311) and are connected together, 

 forming the doubly-concave outer wall of the 

 tooth. The outer wall is marked by a vertical ridge 

 formed at the concrescence of the two tubercles. 

 The outer wall possesses therefore two concavities 

 and is W-shaped (in the worn tooth. Fig. 312, 

 a, c). Tlae intermediate tubercles are also crescentic with their concavities 

 outwards. Tlie anterior intermediate tubercle is connected with the anterior 

 part of the outer wall and the posterior intermediate with the middle part. 

 These in part longitudinally and in part transversely du'ected crescents 

 of the intermediate tubercles form in the worn tooth the laminae of the crown 

 (Fig 312 b^ (P-) ; the spaces between them and the outer wall are filled 

 with cement. The two inner tubercles (vertical pillar) remain tubercular, 

 but both are connected by ridges with their respective intermediate 



tubercles. The anter- 

 ior inner tubercle (Fig. 

 312, b) is the larger. 

 All these tubercles are 

 formed of enamel sur- 

 rounding dentine and 

 the valleys between 

 them are filled up with 

 cement which also 

 covers the whole 

 crown of the tooth. 

 As a result of masti- 

 cation these tubercles 

 are worn down and 

 islands of dentine are 

 In consequence of this the 

 by double bands of enamel 



Fig. 312. — A upper, B lower molar of Equus caballus (from 

 Zittel). o antero-external, 6 antero-internal, 6' antero- 

 intermediate, c postero-external, d postero-internal, rfi 

 postero-intermediate tubercle, y posterior internal, fi' i3" 

 anterior internal, o anterior part of anterior outer tubercle 

 of the lower molar. 



exposed surrounded by their enamel, 

 crown of the worn tooth is traversed 



containing dentine between the two laminae of each band and em- 

 bedded in the cement which covers over the whole crown and fills in 

 the valleys (Fig. 312). The enamel on the concave side of the mtermediate 

 tubercles is slightly plaited. In the lower molars there are only the four 

 main tubercles, the intermediate tubercles being absent. The two outer 

 tubercles are crescentic and joined to form the outer wall, but the crescents 



