28 



NATURAL HISTORY OF BIRDS. 



birds. Their number was about forty-nine, a high number for the class; but the most 

 interesting part of the vertebral column is undoubtedly the tail, which was composed 

 of the great number of twelve vertebras. The middle and posterior ones had very 

 long and broad transverse processes, which restricted lateral motion, clearly indicating 

 that the tail was mainly moved up and down, evidently as an aid in diving, the lateral 



FIG. 14. Caudal vertebrae of Hespcrornis. 



motion being confined to the tail as a whole, and performed between the foremost 

 vertebras. The last three or four caudals were firmly fused together, forming a flat 

 plate, analogous to, but quite unlike, the ploughshare bone of modern birds. Thus 

 the tail formed a sort of an oar, similar to a beaver's tail (Fig. 14). 



The shoulder girdle, in its retrograde development, is particularly interesting as 

 showing strong resemblance in many respects to that of the existing dromasognathous 

 birds, especially when we remember that Ilesperornis itself had a palatal structure of 

 a similar type ; for not only is the sternum devoid of a keel, but the long axes of the 



adjacent parts of the scapula and 

 coracoid were parallel, or identical, 

 as shown in the accompanying cut 

 (Fig. 15). The breast bone was 

 thin and weak, with a rounded 

 mesial projection in front, corre- 

 sponding to the manubrium; the 

 posterior margin was quite thin, 

 and had two shallow emarginations. 

 The ribs show only little difference 

 from those of modern birds, and 

 some of them supported uncinate 

 processes. The clavicles were sep- 

 arate, resembling the correspond- 

 ing bones in the embryos of some 



Fi<;. 15. Scapular arch of ffesperorms, reduced. 



modern forms. The coracoids and 



the scapula were quite small. The wing is represented by the rudimentary humerus 

 alone, the other bones having become atrophied. 



The pelvis, though in its general form resembling that of Colymbus, exhibits many 

 features common to that of reptiles, and of several dromreognathous living birds. 

 Most interesting, perhaps, is that the condyloid cup of the hip-joint is closed by bone, 

 except a foramen that perforates the inner wall, entirely unlike the acetabulum of 

 other birds, but resembling that of the crocodiles. The three constituents of the pel- 



