ARTIODACTYLA. 



577 



c | p I ra f , 



replaced ; it may persist for some 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 calcaneurn, 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 7 -shaped 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. Homacodon vagans. A right upper pre- 

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

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



Z II 



P P 



