392 STRUCTURE AND CLASSIFICATION OF BIRDS 



HESPERORNITHES 



Definition. — Large extinct birds of a diver-like form. Skull holorhinal, 

 ■with, supra-orbital impressions. Vomers paired. Quadrate 

 single-lieaded. Sternum without keel. Shoulder girdle platy- 

 coraooidal ; clavicles fully developed. Fore limb represented by 

 humerus only. 



The principal source of information concerning the 

 remarkable genus Hesperornis, from the Cretaceous of North 

 America (to which three species, viz. H. regalis, H. crassipes, 

 and H. gracilis, are assigned), is naturally the magnificent 

 treatise on this bird (and on Ichthyornis) by Mabsh.' 



The bird stood about six feet high, and presented the 

 general form of the diver, to which bird it is now held to 

 come nearest. Maesh, however, compared it more especially 

 with the StrutMones, and it is spoken of by him and by 

 others as an ' aquatic ostrich.' The former view now holda 

 the field. 



There are, nevertheless, various struthious features in 

 the skull, and in other parts of the skeleton, some of 

 which may be held to be due to its loss of the power of 

 flight ; others are not so explicable from the point of view of 

 our actual knowledge of bird structure. 



The skull has the general contours of that of the diver, 

 and, like that bird, has very well-marked furrows for the 

 supra-orbital glands. The nasal bones produce a holorhinal 

 nostril. The figure given by Maesh of the upper surface of 

 the skull is a little suggestive of there having been an ap- 

 pearance of the ossified ethmoid on the surface of the skull, 

 as in some struthious birds and tinamous. The interorbital 

 septum is fairly ossified with a large foramen. The lacrymal 

 is large with a large descending process, which nearly, if not 

 quite, reaches the jugal bar. The pitting of the premaxil- 

 laries led Maesh to the inference that the beak was present, 

 in addition to the teeth, which will be presently referred to. 



The skull has been described as ' saurognathous,' on 



' Odontornithes. Washington, 1880. 



