No. i.] OSTEOLOGY OF PCEBROTHERIUM. 5 1 



The most abundant of the Loup Fork camels is Procamelas, 

 which is represented by several species. In this genus the 

 upper incisor series is as reduced as in the existing forms, but 

 the first and second persist for a somewhat longer time, as Cope 

 has pointed out. The premolars are not reduced in number : 

 the first is implanted by two fangs, though they show a strong 

 tendency to coalesce. The second is likewise small, with a 

 compressed trenchant crown and small anterior basal cusp. 

 Except for its relatively small size this tooth has changed very 

 little from the condition found in PcebrotJierinm. The third 

 premolar differs considerably in the specimen of P. occidentalis 

 figured by Cope from that given by Leidy. In the former it 

 " consists of a small anterior cusp and a long posterior blade. 

 A strong basal cingulum represents the interior crescent " (No. 

 6, p. 332, PI. LXXVI, Fig. 2). This tooth is decidedly the 

 most extended antero-posteriorly of the premolar series. In 

 Leidy's specimen (No. 19, PI. XX, Fig. 1) the third premolar is 

 decidedly shorter and shows a strong tendency to form an 

 internal crescent, by the elevation and extension of the anterior 

 and posterior basal cusps, but which is incomplete in the middle. 

 We have here, therefore, the formation of a tooth very similar 

 to pm. 4, but in quite a different manner. The fourth premolar 

 is of the typical pattern, and need not detain us. The upper 

 true molars differ from those of the earlier genera only in their 

 tendency to form prismatic crowns, and in the greater promi- 

 nence of the anterior and median external buttresses. The 

 median ribs on the outer walls of the external crescents are still 

 distinct. 



In the lower jaw the premolars are undiminished in number, 

 but considerably reduced in size. The first is a small, simple 

 cone, implanted by a single fang. The second is larger and 

 compressed, with indications of internal basal cusps. The third 

 and fourth are much more elongate and compressed ; when 

 viewed from the outer side, they show two concavities separated 

 by a median convexity : internally there are parallel ridges 

 which enclose one or two valleys. The true molars are more 

 hypsodont than in the earlier forms, more flattened on their 

 inner walls, and the fifth lobe of the last one is larger and con- 

 tains a more distinct valley. 



Contemporary with Procamelns is the genus Pliauchenia, Cope, 



