141 2 CLASS MAMMALIA. 



generalised Ungulates is quite evident ; and Professor Cope would 

 derive them from Tillodontia, allied to Calamodon and Psittaco- 

 therium, and accordingly considers that their lower incisors and the 

 corresponding upper ones represent the second of the three typical 

 pairs. This view is, however, not supported by the dentition of 

 the young of the Leporidce. The LeporidcE show certain points of 

 resemblance to Typotherium among the Toxodontia ; while in the 

 entire order distant affinities are indicated to the Proboscidea, as is 

 shown by the characters of the teeth, the acromion of the scapula, 

 and the position of the jugal. 1 



This order is well known from the period of the Upper Eocene 

 (Oligocene), while Decticadapis apparently carries it back to the 

 commencement of the Tertiary. It has not suffered very greatly 

 by the extinction of family types at the present day, although all 

 the gigantic forms have completely disappeared. 



Suborder i. Duplicidentata. — This suborder is characterised 



2 



by the number of the incisors, which in the adult are - , but at birth 



i 



3. The outer pair of upper incisors is very soon lost, the second 

 i 



pair being of small size, and placed directly behind the large first 



pair. The incisive palatal foramina are large and confluent ; the 



fibula is anchylosed to the tibia, and articulates with the calcaneum. 



The enamel-coat of the incisors is not entirely confined to their 



anterior surface. 



Family LeporidcE. — In this family the number of cheek-teeth is 



Pm. -, M. - ; these teeth being rootless, with transverse enamel- 



2 3 



folds. The clavicles are imperfect, and the fore limbs shorter than 

 the hind ones. This family includes at the present day only the 

 cosmopolitan genus Lepus, comprising the Hares and Rabbits. 

 Existing species of that genus have left their remains in the caves 

 of Europe and Brazil ; while extinct ones are found in the Upper 

 Pliocene of France (Z. Lacosti), in the Siwaliks of India, and in the 

 John Day Miocene of North America. Palceolagus from the Mio- 

 cene of the latter country is closely allied to Lepus ; while Panolax 

 from equivalent strata in New Mexico has only a single plate in the 

 first upper true molar. 



1 The Rodents differ from the Proboscidea in having a discoidal in place of a 

 zonary placenta ; but it has recently been found that in the Mouse there are 

 traces of this discoidal placenta having been derived from an earlier zonary one. 

 The zonary placenta of the Proboscidea is evidently a specialisation of the diffused 

 placenta of the other Ungulates ; and we can now see how a further specialisation 

 of the zonary placenta results in the discoidal one of the Rodents, Insectivores, 

 and Primates. 



