282 THE OSTEOLOGY OF ELOTHEKIUM. 



cies in which these jugal plates are found. Nothing is known concerning their presence 

 or absence in the European reju'esentatives of the genus. Leidy's material gave him no 

 reason to suspect their occurrence in the sjoecies described by him, and he consequently 

 restored the zygomatic arches without them ('69, PI. XVI). Marsh first discovered the 

 processes in a skull of the species named by him E. crassum, and it has sometimes been 

 assumed that they were more particularly characteristic of that form. As a matter of 

 fact, they have been observed in all of the American species of which well-preserved 

 skulls are known, viz., E. mortoni, E. ingens, and E. imperator, and, in all probability, 

 all the American forms, at least, possessed them. 



The lachrymal is a rather large bone and forms nearly half of the anterior boundary 

 of the orbit. On the face it is expanded into quite a large plate, which articulates below 

 with the jugal, in front with the maxillary, and above with the frontal, the long anterior 

 process of which prevents any contact between the lachrymal and nasal. In Hippopota- 

 mus the very short, broad frontal has no anterior process, and so the nasal and lachrymal 

 are connected, as they are also in Sus. Within the orbit the lachrymal is but little 

 extended ; the foramen is single, very small, and placed inside the orbital margin. The 

 lachrymal spine is very low. 



The nasals are narrow, slender and very much elongated. Their greatest width is 

 al the anterior end of the nasal processes of the frontal, and here is also their greatest 

 transverse convexity; from this point they narrow and flatten, both in front and 

 behind. Anteriorly they contract very gradually and terminate in sharp points, with 

 their free ends quite deeply notched. In E. ingens the nasals appeal- to be relatively 

 shorter than in the other species. In Hippopotamus these hones have much the same 

 shape as in Elotherium, but they narrow more abruptly behind the point of greatest 

 width, and their free ends are not notched. In Sax the nasals are truncated posteriorly 

 and in front their free tips project far beyond the borders of the preniaxillaries. 



The premaxillaries are very large and heavy bones, the horizontal or alveolar portion 

 especially so. Posteriorly, this portion is constricted, forming a groove for the reception 

 of the lower canine, expanding again in front to carry the large incisors. The palatine 

 processes are not much developed, the very huge incisive foramina leaving but little 

 space for them ; the spines are Ion-' and slender, extending behind the canine alveolus. 

 The ascending ramus of the premaxillary is low and rises gradually behind, and though 

 broad at first, it rapidly becomes very slender, terminating behind in a fine point. 

 Though these bones in Elotherium have a very different appearance from the immensely 

 enlarged premaxillaries of Hippopotamus, yet both may have been formed by divergent 

 modifications of a common plan. 



The maxillary is greatly extended antero-posteriorly, iu correspondence with the 



