a90 STRUCTUEE AKD CLASSIFICATION OF BIRDS 



canal in Colytnbus ; but the hypapophyses are greatly deve- 

 loped, and in the dorsal region Y-shaped, with widely diver- 

 gent flattened and expanded limbs. None of the dorsal 

 vertebrae are fused. Six ribs reach the sternum in the two 

 grebes mentioned ; eight or nine in the divers. The sternum 

 is one-notched ; in the grebes it has in addition a median 

 triangular notch. There is no anterior spine to the sternum 

 in the grebes. The procoracoid is moderately large and 

 hooked in the divers, absent in grebes. The pelvis is very 

 elongated and compressed, as in Hesperornis, but the ischia 

 are not free from the ilia, as in that bird. 



In the skeleton of the leg the most conspicuous feature 

 is the highly developed cnemial crest. The patella too is 

 very large in the grebes, but not in Colymbidse, where, indeed, 

 it is not ossified. In these particulars the grebes resemble 

 Hesperornis. The principal skull characters have been 

 already mentioned in the definition of the group. In addition 

 to these matters the strongly marked temporal fossae may be 

 mentioned, which nearly meet on the upper surface of the 

 skull. They are not so well marked in the small Podicipes 

 minor. In the divers are strongly marked furrows for the 

 supra-orbital glands ; this is not the case with the grebes. 

 The former have also a single median occipital foramen 

 above the foramen magnum. The ectethmoids exhibit the 

 bullate form so characteristic of the Anseres. 



It may be useful to state in a tabular form the principal 

 characters of the grebes and divers.'- 



Muscle 

 Formula 



Grebes . j BX- 

 Divers . ! ABX + 



Carotids 



Cerv. 

 Veit. 



No. of 

 Compl. 

 ■Ribs 



1 21 ' Ankylosed 1 4-6 



2 I 15 ! Not ank. I 8, 9 



Semi- 

 membra- 

 nosus 



These characters appear distinctly to point to the more 

 modified structure of the grebes. There are reasons (p. 165) 

 for regarding a small number of cervical vertebrae and a 



' The extinct Colymboides (C. mmttfjis, M.-Eb. ; C. anglicus, Lydekkee), of 

 which the former is known by a humerus, the latter by a coraooid, and possibly 

 a piece of sternum, is said to combine the characters of grebes and divers. 



