12 I*rof. Nicholson — Progress in Palceontology. 



The great order of the Ungulates has of late years been vastly 

 added to by the discovery of entirely new forms, or of specimens 

 more fully elucidating the structure of previously known forms. I 

 can only advert to one or two points in this connexion. One of 

 the most striking of Marsh's many discoveries is that of the two- 

 horned Rhinoceros to which he gives the name of Diceratherium. 

 This remarkable form is from the Miocene of Oregon ; it differs 

 from all the known two-horned Ehinoceroses in the fact that the two 

 horns are placed transversely and symmetrically upon the nasal 

 bones. In this respect, indeed, the genus diifers from the whole 

 group of the Perissodactyle Ungulates. Another discovery by Marsh 

 is that Coryphodon, which is found in the Eocene of both Europe and 

 America, though essentially Perissodactyle, possesses five toes to the 

 foot, and thus differs from all other known Ungulates except Oro- 

 Jiippus. Of the extraordinary series of forms by means of which the 

 living genus Equus is connected with the Eocene Eahippus and Oro- 

 hippus I need scarcely remind you, as the principal facts in this 

 connexion are now widely known. Another, though less complete, 

 series of connected forms is now known by which the genus Dicotyles 

 is linked on to the Eocene EoJiyus ; and a third series exists by 

 which the aberrant Camelidce of to-day can be traced back to forms 

 which agree with the typical Ungulates in having a full series of 

 incisor teeth (e.g. Poehr other ium and Protolahis). 



Allied to the Ungulates in many respects, and forming a group 

 intermediate between them and the Proboscideans, are the large 

 Eocene Mammals which Marsh has raised to the rank of a distinct 

 order under the name of Dinocerata. The fore-feet are five-toed, 

 and the limbs are like those of the Elephants ; but the most 

 remarkable characters of these animals are those of the cranium. 

 As regards the dentition, the upper jaw is destitute of incisors, but 

 there is a pair of huge tusk-like canines, directed downwards, and 

 there are six small molars on each side. In the lower jaw are six 

 incisors, small canines, and twelve molars and preemolars. The 

 armature of the head is also very peculiar. The upper jawbones 

 carried each a pair of small processes, probably of the nature of 

 horn-cores ; the nasals carried two similar but smaller cores ; and 

 the froutals are developed into two larger bony processes apparently 

 also intended to be sheathed in horn. There were thus three pairs 

 of horns, similar in structure to those of the hollow-horned Rumi- 

 nants. There does not appear to have been any proboscis, and the 

 brain was proportionately smaller than in any known Mammal, 

 either living or extinct, and even smaller than in some Reptiles. 

 We cannot, therefore, credit Dinoceras with the possession of a large 

 amount of intelligence. 



Another remarkable group of Eocene Mammals has been raised to 

 the rank of a distinct order under the name of Tillodontia. These 

 animals possess a curious combination of the characters of the Car- 

 nivora, UngnJata, and Rodentia, The general form of the skeleton 

 most closely resembles that of the Carnivores, the skull being in 

 many respects similar to that of the Bears, while the feet are five- 

 toed, with the whole sole applied to the gi'ound, and having ungual 



