RUMINANTS. 531 



ordinary Ruminants, and the two lateral series in the upper jaw 

 converge anteriorly in a greater degree. 



The characteristic complexity of the molar tooth of a Ruminant 

 is manifested by most of the deciduous series ; but, in the permanent 

 series, only by the three posterior teeth of both upper and lower 

 jaws, which are the true molars ; the three first, or premolars, 

 having more simple crowns than those which they displace. The 

 complexity in question is the result of peculiar folds of the 

 formative capsule, some of which are longitudinal or project 

 inwards from the sides of the capsule and form peninsular folds of 

 enamel upon the grinding surface of the tooth, whilst others 

 depend vertically from the summit of the matrix into the body 

 of the tooth, and form islands of enamel when the crown begins 

 to be worn (PL 134). Of the longitudinal folds two, in the 

 upper true molars, are external, broad but shallow, and often 

 sinuous (fig. 1, 0, o), and one is internal, narrow and deep (ib. i) 

 extending quite across the summit of the crown of the tooth, as 

 in figs. 6 &: 7, and decreasing in depth towards the base of the 

 crown ; the corresponding fold of enamel in the completed tooth 

 accordingly extends more or less across the crown, from within 

 outwards, as the tooth is less or more worn. The crown of the 

 tooth is thus divided into two lobes, placed one in front of the 

 other; the inner side {i i, fig. 3) of each lobe being convex, the 

 outer side (o o) concave but in a minor degree, and usually 

 sinuous or with a convex prominence in the middle (fig. 3 & 4). 

 The vertical folds of the capsule, two in number extend one into 

 each lobe, and in the upper molars are concave towards the outer 

 and convex towards the inner side of the lobe, which is thus 

 divided into two semi-cylindrical bodies or lobules (o, i, fig. 6), with 

 crescentic summits, as seen in a transverse section or on the 

 grinding surface of the tooth ; each lobule consisting of a middle 

 body of dentine {od, id, fig. 5) and an outer coat of enamel and 

 thin cement; the midspace between the lobules, answering to the 

 centre of the vertical fold, is filled partly by cement, and com- 

 monly contains portions of the vegetable food ; it is inclosed by a 

 crescentic island of enamel (figs. 3 & 4, e e) ; the whole 



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