606 UNGULATES. 



series, their crowns being quite crossed by the transverse valle)% 

 which divides the grinding surface into two principal ridges. Cuvier 

 well observes that, as the last true molar is not more complex than 

 the others, so neither is the last of the deciduous series more complex 

 than the premolar which displaces it. In the lower jaw two of the 

 deciduous molars present the narrow elongated subtriangular crown, 

 which is repeated by the first only of the permanent series ; the 

 third deciduous molar had the typical square double ridged crown. 



222. Lophiodon. — Although the analogous instances(l) are so 

 numerous as almost to establish a law, yet it was with unabated 

 interest that I found in the Lophiodon another example of the 

 retention throughout life, by an ancient extinct species, of a charac- 

 ter which is transient and limited to the immature period of existence 

 in its modern representative. The Lophiodons which rank amongst 

 the most ancient of Pachyderms (2), had the same numerical dental 

 formula as the Tapirs ; but the compressed and more simple form 

 of the crown was retained in the three premolars of the lower jaw, 

 the square-shape and double transverse-ridged structure being 

 established only in the three true molars. 



In the upper jaw both the first and second premolars resemble 

 the first premolar of the Tapir, having the outer wall of the crown 

 divided into two demi-cones, and the 'inner part traversed by an 

 oblique ridge continued from the cone which rises from the inner 

 angle of the crown, and the anterior angle of the outer wall : this 

 ridge is higher and more simple than in the Tapir ; the valley on 

 each side is bounded by a basal ridge. In the remaining molars 

 the outer wall of the crown presents three demi-cones, in con- 

 sequence of the great size of the angle of the anterior ridge: 

 the inner side developes two cones which are continued by parallel 

 oblique ridges, one to the anterior outer cone, the other to the 

 interspace between the middle and posterior cone. The intervening 

 valley stops at the base of the middle outer cone which is the highest, 

 and does not cut quite across the crown in any of the true molars : 



(1) Borcatherium, p. 530, Anoj)lotheru:m, p. 523, Machairodus, p. 494, &c. 



1.2) The Tapirus prisms of the older pUocene or the miocene formations of Germany and 

 France, had the same essential dental characters as the existing Tapirs ; but the extinct species 

 first appears at a more recent period than the Lophiodonts of the eocene marls and clays. 



