542 Comparative Morphology of Chordates 



divers, also toothed, having apparently useless vestiges of wings. (3) 

 Numerous diversified types of more or less capable fliers, all short- 

 tailed and toothless; along with them, some aquatic birds having 

 wings more or less reduced — poor fliers or incapable of flight; a few, 

 mostly very large birds, having wings reduced below the level of flight 

 or entirely lacking and, in most cases, legs long and strongly built for 

 running; all birds in this third period are short-tailed and toothless. 



Classification 



The classification of birds is less satisfactory than that of the other 

 Classes, partly because of the dearth of fossil material and also because 

 the diversification of modern birds is so extreme. Comparison of birds 

 of two types is likely to reveal a complex of resemblances and differ- 

 ences from which it is difficult to select those which should be regarded 

 as most significant for purposes of classification. This is true of even 

 the main subdivisions of modern birds. It has long been the common 

 practice to separate birds into two groups — Carinatae, flying birds 

 with a well-developed keel on the sternum; and Ratitae, running 

 birds having reduced wings, the carina poorly developed or lacking, 

 and the terminal region of caudal vertebrae usually not fused into a 

 pygostyle. ("Ratitae" — from the Latin ratis, a raft — has nautical 

 significance, referring to the flat-bottomed keelless sternum.) This 

 grouping, however, is not satisfactory because there are some carinate 

 flying birds (the South American tinamou) which, on the basis of other 

 characteristics, seem to be much more closely related to the Ratitae 

 than to modern carinate birds. This difficulty arises from the fact that 

 the carina is readily subject to adaptive change corresponding to 

 change in the locomotor activities of the animal — that is, not in the 

 individual, but in the long course of phylogeny. It is apparently not a 

 stable or "conservative" character and not trustworthy for determina- 

 tion of long-range relations. 



Probably more confidence may be placed in the structure of the 

 skull, whose form is to a large extent not directly related to motor 

 activities. In some birds the skull follows the reptilian pattern much 

 more closely than in others. The structure of the roof of the mouth 

 (palate) and the upper jaw has come to be regarded as especially 

 significant. In some birds the vomers (or "prevomers": see p. 125), a 

 pair of bones lying in the median region of the anterior part of the 

 palate (Fig. 375, center), are large and each one articulates with the pal- 

 atine and pterygoid, which are rigidly joined together. These relations 

 are commonly found in less specialized reptilian skulls. In other birds 

 the vomers are relatively small, usually very narrow and fused, or 

 sometimes lacking, and they may articulate with the palatines but 



