OREODON. 39 



wliere they conjoin in each tooth, form prominent columns, not robust and con- 

 vex, as in Hyopotamus, but antero-posteriorly compressed and rather abruptly ex- 

 panded near the base of the crown, where they are more or less connected by 

 intervening portions of a basal ridge. In some specimens, however, this ridge is 

 obsolete, more especially at the postero-external lobe. (II. 1, 3 ; III. 1, 2.) 



In two specimens under investigation, the basal ridge externally is well developed, 

 strong, and rough at the margin ; and it exhibits a tendency to extend itself on the 

 outside of the projecting columns, as is indicated by a roughness of the enamel. 

 (111.1,2.) 



As in all ruminants, there is a disposition in the postero-external lobe of the 

 molars to form a posterior column or fold, which, however, to a great extent, 

 except in the case of the last molar, remains aborted ; and which, in the external 

 view of the jaw, is concealed by the anterior column of the antero-external lobe. 



The outer faces of the external lobes of the molars between the columns are 

 transversely concave, slightly prominent in the median line ; and they incline at an 

 angle of about 40°. The inner faces are nearly vertical, but they incline slightly 

 outward, and are angularly convex. 



The internal lobes appear broader transversely than the external, because their 

 outer face becomes confluent at the basal third of the inner face of the latter. 



The outer face of the internal lobes is concave, and very slightly prominent in 

 the median line ; and it inclines to about the same extent as the corresponding 

 surface of the external lobes. The inner faces are not quite as angular in their 

 convexity as those of the external lobes. 



The summits of the latter are confluent at the apex of the median outer column, 

 but those of the internal lobes are not confluent. The anterior extremity of the 

 summit of the postero-internal lobe ceases abruptly, and it is included between the 

 anterior half of the inner face of the postero-external lobe, and the posterior extre- 

 mity of the summit of the antero-internal lobe, which latter extremity bends for- 

 ward to the posterior part of the inner face of the antero-external lobe, and then 

 terminates abruptly. 



Portions of a basal ridge, which are sometimes more or less excavated, and have 

 an irregular crenulated margin, usually exist at the bases, anteriorly and poste- 

 riorly, of the internal lobes. Occasionally they are continuous around the base of the 

 postero-internal lobe of the second and third true molars, but more frequently upon 

 the latter alone than upon both. Between the lobes internally, the ridge sometimes 

 forms a single tubercle, simple and obtuse, or excavated. 



In the trituration to which the true molars are subjected in mastication, the 

 summits of the anterior lobes suffer at first more than those of the posterior, and 

 the internal more than the external ; but, in course of time, the abrasion is nearly 

 equalized over the grinding surface. 



"When the dentinal substance is first exposed by the removal of the enamel 

 summits of the external lobes, the surface presents the form of the letter W, or of 

 two crescents, confluent where contiguous. At the same period, the inner lobes 

 present distinct broader crescents of exposed dentinal substance. As the attrition 

 proceeds, the latter crescents increase in breadth, and also become continuous. 



