EURHIPIDURjE. 31 



SUB-CLASS IV. EURHIPIDURJE. 



Eurhipidurae, the term applied by Gill to these birds in contradistinction to 

 Saururae, means ' birds provided with a normal fan tail,' and expresses very well the 

 general feature of this sub-class, namely, a feather tail, the base of which, formed by 

 the shafts, practically starts from a single point, spreading out at the end like a fan, 

 notwithstanding the fact that not a few birds of this division are deprived of this 

 'external' tail, as, for instance, some Struthionine birds and the grebes, since this 

 absence is only due to a retrograde development of forms which were as * eurhipidu- 

 rous ' as the rest of them. More trenchant is the corresponding internal or anatomical 

 arrangement of the caudal vertebrae. These are reduced in number, shortened, and 

 the last ones united into a single bone, the pygostyle, the total length of the tail 

 proper being much less than that of the rest of the body. But while these characters 

 are also shared by the Ichthyornitb.es of a foregoing sub-class, the members of the 

 present one have no biconcave antesacral vertebrae, nor teeth in the jaws. 



The Eurhipidurae embraces all the living birds, besides a number of extinct forms 

 from the more recent geological formations, consequently all birds known to constitute 

 the class until less than a quarter of a century ago. 



SUPER-ORDER I. DROM^EOGNATH^. 



Two peculiarities in the anatomical structure of the birds of this group separate 

 them at once and rather widely from the rest of the Eurhipidurce, namely, the 

 dromaeognathous character of the palate, and the relations of the pubic elements, 

 of which the ilium and the ischium are not united behind, and do not enclose a sciatic 

 foramen as in both Impennes and Euornithes. These characters are combined with 

 several others, and seem to us to be of more importance than the presence or absence 

 of a keel on the sternum. The absence of that element in some forms of the present 

 group may be due to abortion and reduction. This is the most probable explanation, 

 though it may not be the true one. But in the latter case the birds with keeled 

 sternum must have sprung from forms not having a keel, as we can hardly assume 

 that Ratitae and Carinatae sprung from the ancestral reptilian stock independently of 

 each other. On the other hand, there is ample pi-oof that several Euornithes have 

 become nearly keelless by reduction (Didus, JVbtornis, Hdbroptihis), so there seems 

 not to be any reason why it should not be lost altogether. Again, the plumage and 

 rudiments of rectrices in several Ratitae (cassowary, Apteryx), strongly point towards 

 their reduction from flying ancestors, or, to use Wallace's words, " render it probable 

 that the Struthious birds do not owe their imperfect wings to a direct evolution from a 

 reptilian type, but to a retrograde development from some low form of winged bird. 

 . . . We may be sure that birds acquired wings and feathers and some power of 

 flight before they developed a keeled sternum, since we see that bats, with no such 

 keel, fly very well." 



The Dromaeognathae, which once formed the prevailing constituents of the earth's 

 bird-fauna, are now only few in number. Many of their members have been extin- 

 guished during the present geological period, and several are soon to follow. The 

 present era is that of the Euornithes. 



Not before the theory of evolution had become fairly established was the impor- 



