GEOLOGICAL SUCCESSION OF ORGANIC BEINGS 115 



ing manner that many of the fossil mammals discovered 

 by him in Attica serve to break down the intervals be- 

 tween existing genera. Cuvier ranked the Euminants and 

 Pachyderms as two of the most distinct orders of mam- 

 mals: but so many fossil links have been disintombed 

 that Owen has had to alter the whole classification, and 

 has placed certain pachyderms in the same sub-order 

 with ruminants; for example, he dissolves by gradations 

 the apparently wide interval between the pig and the 

 camel. The Ungulata or hoofed quadrupeds are now 

 divided into the even-toed or odd-toed divisions; but 

 the Macrauchenia of South America connects to a cer- 

 tain extent these two grand divisions. No one will deny 

 that the Hipparion is intermediate between the existing 

 horse and certain older ungulate forms. What a wonder- 

 ful connecting link in the chain of mammals is the 

 Typotherium from South America, as the name given 

 to it by Professor Gervais expresses, and which cannot 

 be placed in any existing order. The Sirenia form a 

 very distinct group of mammals, and one of the most re- 

 markable peculiarities in the existing dugong and laman- 

 tine is the entire absence of hind limbs without even 

 a rudiment being left; but the extinct Halitherium had, 

 according to Professor Flower, an ossified thighbone "ar- 

 ticulated to a well-defined acetabulum in the pelvis,'' 

 and it thus makes some approach to ordinary hoofed 

 quadrupeds, to which the Sirenia are in other respects 

 allied. The cetaceans or whales are widely different 

 from all other mammals, but the tertiary Zeuglodon 

 and Squalodon, which have been placed by some nat- 

 uralists in an order by themselves, are considered 

 by Professor Huxley to be undoubtedly cetaceans, 



