108 AFFINITIES OF EXTINCT SPECIES. [Chap. XI. 



horse and certain older ungulate forms. What a won- 

 derful connecting link in the chain of mammals is 

 the Typotherium from S. 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 

 remarkable peculiarities in the existing dugong and 

 lamentin is the entire absence of hind limbs without 

 even a rudiment being left ; but the extinct Halitherium 

 had, according to Professor Flower, an ossified thigh- 

 bone " articulated 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 Zeug- 

 lodon and Squalodon, which have been placed by some 

 naturalists in an order by themselves, are considered by 

 Professor Huxley to be undoubtedly cetaceans, " and to 

 constitute connecting links with the aquatic carnivora." 



Even the wide interval between birds and reptiles 

 has been shown by the naturalist just quoted to be 

 partially bridged over in the most unexpected manner, 

 on the one hand, by the ostrich and extinct Archeo- 

 pteryx, and on the other hand, by the Compsognathus, 

 one of the Dinosaurians — that group which includes 

 the most gigantic of all terrestrial reptiles. Turning to 

 the Invertebrata, Barrande asserts, a higher authority 

 could not be named, that he is every day taught that, 

 although palaeozoic animals can certainly be classed 

 under existing groups, yet that at this ancient period 

 the groups were not so distinctly separated from each 

 other as they now are. 



Some writers have objected to any extinct species, 



