466 SIR. F, E. BEDBABD OX ISTERCESTEA IX BIRDS. [Ma 4, 



The hypocentra — as may be seen in various species with free 

 intercentra — are intercentra which are attached to the summit of 

 short outgrowths of the centra. Thus the coalesced hypapophysis 

 is really intercentrum plus this median central outgrowth. In 

 several cases where there are no free intercentra, but only the 

 hypocentra, it would be difficult to assert that the latter are really, 

 as has been stated, partly produced by the intercentra! discs of bone. 

 They appear to belong so entirely to the centrum. But that this 

 is not the case can be readily seen from such examples as that 

 shown in the drawing (woodcut, fig. 1), where the free intercentra 

 gi-adually come to be more and more thoroughly articulated with 

 the centrum behind. 



The intercentra when present as free structures appear in- 

 variably to increase in size from before backwards. They often 

 begin as little more than mere granules of bone, which, as they 

 are often easily detachable, may have been missed, though really 

 present. They are especially loosely attached, and therefore liable 

 to be lost, in the Auks. 



So far as can be judged from the facts which are briefly stated 

 in the following pages, the existence of free intercentra is not 

 universal but very general among birds. There are, however, 

 groups, such as the Cuckoos and Columbse, in which these struc- 

 tures are not present as independent bonelets. There are also some 

 birds in which they do not appear to exist at all ; in which, indeed, 

 there are no hypapophyses which may be presumed to be these 

 structures, but this is rare. 



They are obviously most prevalent in water-birds, including, 

 however, the land Limicolae, which are so nearly akin to the Gulls 

 and Auks. They are rarest among arboreal birds, such as Parrots, 

 Pigeons, Picarian and Passerine birds ; but occur in the possibly 

 archaic Opisfhocomus. In the reduced tail of the Struthiones 

 intercentra do not occur plentifully. Parker, as has been already 

 mentioned, finds two in Apteryx. But Mivart \ in his elaborate 

 description of the axial skeleton of the Struthiones, does not appear 

 to have met with them. There does not appear to be a very 

 definite connection between the presence of intercentra and the 

 lowness of the place of the bird in the series ; but, on the other 

 hand, there does, with some exceptions, seem to be some relation- 

 ship between the length of the tail and the existence of free inter- 

 centra. It is not known to what extent these structures existed in 

 most of the important types of extinct birds ; but Marsh found 

 two free intercentra in Hesperornis ^. 



AWSEEES. 



In Biziura lobata (fig. 1) the intercentra are especially well 

 developed. There is a small oval nodule 3 mm. long between the 

 last sacral and the first free caudal, a rather larger one between 

 this and the following vertebra, and in the next interval a large 



' " On the Axial Skeleton of the Ostrich," Zool. Trans, viii. ; and " On the 

 Axial Skeleton of the Struthionidte." ibid. x. 



^ See his great work upon the " Odontornithes." Washington, 1880. 



