850 ANATOMY OF VERTEBEATES. 



but there is a rudiment of d 1 in the embryo Fallow-deer, and in 

 one of the most ancient of the extinct Ruminants (Dorcatherium, 

 Kaup) the normal number of premolars was fully developed. 



The characteristic complexity of the Ruminant grinder, fig. 

 271, is seen, in the permanent series, only in 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 plications of the formative capsule, some of 

 which are longitudinal, or project inward 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. Of the longi- 

 tudinal folds, two in the upper true molars are external, broad, 

 but shallow, and often sinuous, and one is internal, narrow, and 

 deep, extending quite across the summit of the crown of the 

 tooth, and decreasing in depth toward the base of the crown. 

 The corresponding fold of enamel in the completed tooth, ac- 

 cordingly, extends more or less across the crown, from within 

 outward, as the tooth is less or more worn. The whole circum- 

 ference of this complex molar is also invested by a coat of enamel 

 and a thinner layer of cement. In some Ruminants, e.g. Ox, 

 Deer, and Giraffe, a small vertical column, fig. 271, m, is de- 

 veloped at the internal interspace of the two lobes of one or more 

 of the upper true molars, varying in height, and rarely reaching 

 the summit of the new-formed crown, but longest in the Bovidce. 

 Different genera of Ruminants also differ in the depth and sinu- 

 osity of the two outer longitudinal folds, f, and in the depth and 

 complexity of the two vertical folds, h 9 1, which likewise are united in 

 some species by a longer common base than in others, producing 

 thereby a continuity of the enamel, and complete anteroposterior 

 bisection of the grinding surface during a longer period of attri- 

 tion. The molars of the Camel present the most simple con- 

 dition of the Ruminant type of these teeth ; the transverse fold 

 dividing the crown being short, the dentine of the two lobes soon 

 forms a continuous tract. The common base of the crescentic 

 vertical folds of the capsule being likewise short, the enamel 

 islands are soon separated from each other. They include a 

 shallow or narrow crescentic cavity, with a simple but slightly 

 sinuous contour. The two outer shallow longitudinal depressions 

 of the crown have no middle rising ; and there is no columnar 

 process at the interspace of the two inner convexities. 



