•JWL6 Marsh — A New Order of Extinct Mammals. 



The large collection of the remains of this group secured by 

 the writer contains almost every part of the skull and skeleton, 

 pertaining to many individuals. Among these are specimens 

 which agree closely with the types described and figured by 

 Cope, as well as a number of others that are evidently quite 

 distinct. Besides Meniscotherium, a second genus is repre- 

 sented, which may be called Hyracops, and its typical species 

 is described below. This genus is more specialized than 

 Meniscotherium, and may be from a somewhat higher horizon 

 of the lower Eocene, but the two genera are found in the same 

 region, and may perhaps occur together. 



An investigation of all the specimens represented in this 

 collection of the writer makes it evident that they pertain not 

 merely to a distinct family, but to a separate order, which may 

 be called the Mesodactyla. The skull and teeth resemble 

 strongly those of the Vngulata, especially some of the early 

 Perissodactyles, but the limbs and feet prove these fossils to be 

 quite distinct from any hoofed animals hitherto known. 



The skull of Hyracops, in its general form, resembles that 

 of DideVphys. The premaxillaries do not reach the frontals. 

 The latter are wide between the orbits, and there is a small 

 postorbital process, behind which the skull is much constricted. 

 There is a strong sagittal crest. The malar reaches the lachry- 

 mal. The palate is complete, and the posterior nares extend 

 forward to the penultimate molar. The brain was of moder- 

 ate size, with the olfactory lobes and the cerebellum especially 

 developed, and not at all covered by the cerebral lobes. 



The dentition of the Mesodactyla is of special interest. 

 The teeth are brachydont, the molar series lophodont, with 

 external and internal crescents and intermediate cusps, with 

 deep valleys. In one genus, with which the type specimen of 

 Meniscotherium best accords, the premolars are unlike the 

 molars. In the genus Hyracojps, the last premolar is nearly or 

 quite like the molars. 



A striking feature in the dentition of both genera is the suc- 

 cession of the teeth. The first series of incisors, canines, and 

 premolars, appears to be retained for a long period. This is 

 certainly the case with the premolars, which continue in use 

 until long after the three true molars have come into service. 

 This fact confirms the recent researches of Kiikenthal on 

 DideVphys, and indicates that the supposed true molars may 

 belong to the first dentition. 



The vertebrae have their articular faces nearly or quite flat, 

 and in the cervical region, these are oblique. The sacral 

 vertebras are firmly coossified, and are three in number in 

 Meniscotherium, while in Hyracops there are four. The 

 sternal bones are flat. 



