CHARACTERS OF IliEMATOTIIERMA. 13 



In all the Cursorial genera the sternum is devoid of keel. 



Struthio is the only genus of birds in which 

 the toes are reduced to two, fig. 11. 



In like manner the web-footed order is an 

 artificial one, including derivatives from dif- 

 ferent natural groups or types ; and the same 

 may be said of the order including the birds 

 that have the leo;s lono; and naked above the 



tarsal joint. ^ ^ cursorial foot of Ostnch. 



Derivatively the class of Birds is most 

 closely connected with the Pterosaurian order of cold-blooded air- 

 breathers. In equivalency it is comparable rather with such a 

 group than with the RejJtilia in totality, or with the Marnmalia ; 

 and, hence, the corresponding inferiority of value of the avian 

 ^ orders ' to the subdivisions so called of those larger classes. 



In relation to time, indications of Aves date as far back as those 

 of Pterosauria, in the ' ornithichnites ' or foot prints of the New- 

 Red Sandstones, for example.^ The lithographic slates of a later 

 mezozoic period have revealed a true feathered bird,^ wanting 

 only the adaptive modification of the caudal vertebro3 charac- 

 teristic of all neozoic birds, even those of the oldest tertiary 

 strata, in which fossil remains of representatives of nearly all the 

 present orders of Aves have been found.'^ The most recent in- 

 stances of extinction of species are of the birds that have lost the 

 power of flight; as, e.g., the gigantic ^loas {Di?iornis, Palapten/x, 

 Aptoi'nis, Ciiemiornisy of New Zealand ; the equally gigantic 

 Epyornis of Madagascar ; the Dodo (^Didus) of the Mauritius ; 

 the Solitaire {^Pezopliaps) of Rodriguez ; the Gare-fowl (^Alca 

 impennis) of Northern shores or islands. 



Notwithstanding the characteristic powers of locomotion of the 

 class generally, it is amenable, most suggestively, to laws of geo- 

 graphical distribution and limitation. 



' xvn-. pp, 5 and 324. - xv. ^ CLr. and xviii". p. 549. 



' XV1-. 



