130 THE MAMMALIA OP THE DEEP EIVEK BEDS. 



The mandible is most like that of Eporeodon, but with some changes in the 

 direction of Merychyus. Owing to the procumbency of the incisors and their alveoli, 

 the outline of the chin, when viewed from the side, is seen to be strongly concave. 

 In Eporeodon the posterior margin of the angle and ascending ramus is regularly 

 rounded and projects far back of the condyle, while in Merychyus zygomaticus this 

 margin is nearly straight and vertically directed and extends but little back of the 

 condyle, from which it is separated by a notch. In Mesoreodon the shape of this 

 region of the mandible is intermediate between these two extremes. The coronoid 

 is short and slender, the sigmoid notch deep and widely open, and the condyle is 

 much extended transversely. 



By a happy accident the hyoid (PI. Ill, Fig. 29) is preserved almost intact in 

 one of the specimens and in its natural position. This apparatus in some respects 

 differs from that of any known artiodactyl and agrees better with the hyoid of certain 

 perissodactyls. 'No doubt can exist as to the proper reference of the specimen, as is 

 demonstrated by its connection with the skull, which was that of a large male, as 

 indicated by the robust canines. 



The tympano-hyal is a short, stout, cylindrical bar, which is inserted into a de- 

 pression upon the outer side of the auditory bulla. The stylohyal forms a long and 

 broad (antero-posteriorly) but thin and very much compressed bar, which expands 

 at the proximal end, but this portion is fractured, so that its exact shape cannot be 

 determined. Except for this proximal expansion, the bone is of almost uniform size 

 throughout. The epihyal is well ossified and relatively longer than in the sheep ; it 

 is narrower and somewhat thicker than the stylohyal and tapers distally. The cerato- 

 hyal is also better developed than in the modern ruminants ; it is of a curious, paddle- 

 like shape, slender and rounded where it joins the epihyal and expanding into a 

 rounded blade posteriorly, where it is applied to the basihyal ; it is not ankylosed with 

 the latter. The basihyal is unlike that of any artiodactyl, with which I have been 

 able to compare it, and much more resembles that of the horse. In shape it is nar- 

 row, depressed and thin and curved backward ; i. e., with the concavity towards the 

 front. Its great peculiarity, for an artiodactyl, consists in the presence of a glosso- 

 hyal process, which is given off from the middle line of the hinder border. This 

 process is much shorter proportionately and more curved backward than in the 

 horse, and is compressed antero-posteriorly instead of laterally. I can find no other 

 artiodactyl in which this process occurs. The thyrohyals are ankj'losed with the 

 basihyal, at which points they form slight, club-shaped swellings ; they are slender, 

 rounded, arched somewhat anteriorly and are of unusual relative length, being nearly 

 as long as the stylohyals, though of altogether different shape. 



