ARTIODACTYLA. 



577 



p 4 W TT, 



replaced ; it may persist for sone time. There is no 

 alisphenoid canal and the nasals are not expanded posteriorly. 

 The number of dorso-lumbar vertebrae is always nineteen. 

 The femur is without a third trochanter, the fibula articulates 

 with the calcaneum, and the facets upon the distal face of 

 the astragalus for the navicular and cuboid are sub-equal, 

 and both its distal and proximal surface are pulley-like. 



These are the differential characters of the living; members of 

 the sub-order, and it is upon their application to the imperfectly 

 known extinct forms that the systematic position of these must 

 depend. The digits are very commonly reduced in number ; the 

 limit of the reduction being found in the more specialised Rumin- 

 ants in which digits 3 and 4 alone persist, all trace of the others 

 having disappeared. 



The dentition consists typically of 44 teeth, i 

 but there is a tendency a 



towards the suppression 

 of the upper incisors, and, 

 as already stated, the first 

 tooth of the premolar 

 series is probably the 

 long -persistent first milk 

 molar. The premolars 

 are usually simpler than 

 the molars (Fig. 296). 



In the so-called primitive forms the grinding teeth are low- 

 crowned (brachyodont) and the tubercles are conical (bunodont) ; 

 the latter being in two pairs (Fig. 296) with a tendency to the 

 intercalation of an accessory tubercle in the upper molars between 

 the tubercles of either the anterior or the posterior pair. There 

 is almost always a heel (talon) on the last molar of the lower jaw 

 (Fig. 296 B). In many forms the tubercles become V-phaped or 

 semilunar and are connected by basal ridges ; in this way the 

 teeth pass through a stage which may be called buno-lophodont 

 or buno-selenodont to selenodont, the form which is eminently 

 characteristic of the Ruminantia (Fig. 297 ) . In selenodont forms 

 the concavity of the semilunar tubercles are directed outwards 

 in the upper and inwards in the lower jaw ; and in the upper jaw 

 the two outer tubercles are usually united, forming the outer wall 

 of the tooth, and there is a projecting ridge along the wall at the 



Fig. 296. — Homacodun vagans. A right upper pre- 

 molars 2-4, molars 1-3 ; B right lower premolais 

 3 & 4, molars 1-3 (after Marsh, from Woodward). 



