354 MAMMALIA. 



There are other well-known forms from the same localities, and also from 

 the Upper Eocene Phosphorites of Quercy, France. 



The Xiphodontidae are another family of these primitive 

 forerunners of the ruminants from the Upper Eocene, also 

 apparently confined to the western European area. Some of 

 them are the largest and most slender of the artiodactyls 

 of their age, and the only feet hitherto discovered are two-toed, 

 with mere rudiments of the lateral digits. 



Xiphodon. The dentition is complete, forming a regular series, without 

 any conspicuous diastema or any enlargement of the canine. The upper 

 molars exhibit a supplementary fifth cusp between the ordinary two 

 anterior cusps ; the hindermost lower molar is produced into a small third 

 lobe posteriorly. Pm. 1 to 3 are much extended antero-posteriorly. The 

 limb-bones are long and slender, and the lateral digits (nos. u. and v.) of 

 both feet are represented solely by the upper end of the metapodials. The 

 typical species is Xiphodon gracilis from the Upper Eocene (gypsum) of 

 Montmartre, Paris, the contemporaneous lignites of De'bruge, and the 

 Phosphorites of Quercy; also from equivalent deposits in Switzerland. 

 This animal seems to have attained a length of less than a metre, and 

 it is associated with both larger and smaller species. Dichodon (fig. 203) 

 and Amphimeryx are contemporaneous closely related genera. 



The third of the primitive families already mentioned as 

 requiring notice, is that of the Oreodontidae, apparently 

 confined to North America, but ranging from the Upper 

 Eocene to the Upper Miocene. These are animals not larger 

 than sheep, with a very long tail, and with four functional 

 toes on each foot. The dentition is usually complete, though 

 the incisors are sometimes rudimentary or even wanting in the 

 upper jaw ; and all the genera exhibit one very peculiar feature, 

 namely, the degradation of the lower canine to the rank of an 

 incisor, while the foremost premolar is enlarged to usurp its 

 function in opposing the moderately large upper canine. Most 

 of the genera are known by the nearly complete skeleton, and 

 two of them, Oreodon and Agriochcerus, are especially note- 

 worthy. 



Oreodon. In the skull the facial and cranial regions are about equal 

 in length, and there is a deep antorbital pit (not a vacuity). The sagittal 

 crest is acute and prominent, while the occiput is a little overhanging. 

 The orbit is separated from the temporal fossa by a slender bar of bone, 

 and the zygomatic aroh is not very robust. The tympanic bullee are a 



