Sjjstematlc Position uf the Sheath-hills. 143 



Skuas, Gulls, Oyster-catchers, and Dromas (Crab- Plover), 

 or, to be probably more exact, they are modifications of these 

 structures, as they were possessed by an ancestral form from 

 which all the groups above mentioned have possibly sprung 

 by discontinuous variations. 



Just caudad of the projecting lacrymals there is a pro- 

 minent sickle-shaped notch with smooth and rounded edges, 

 and this may be converted by a bony bridge into a complete 

 foramen or left incomplete. 



It is interesting to note that in the case of the Skuas this 

 notch may also be bridged across by well-organised osseous 

 connections which appear to be something very distinctly 

 more than ossified ligaments; but this, so far I am aware, only 

 applies to the genus Megalestris, and even in that genus to 

 New Zealand types only. I have not found a skull of 

 Stercorarius in which this notch is converted into a complete 

 foi-amen. 



Thus in both the Chionis and Chionarchus groups, as well 

 as in the Skuas, we find skulls indicating transitions as 

 regards this supraorbital region, from a more generalised to 

 a more specialised condition, unless, indeed, these progressive 

 steps are simply indications of age. 



It would appear, hoAvever, judging from the various 

 localities from which these skulls have been collected, that 

 the eflFects of isolation in this connection cannot be ignored; 

 but until a far greater series of skeletons is available it would 

 be dangerous to draw any conclusions. Attention, however, 

 is especially drawn to the differences presented in the mor- 

 phology of this region in the case of the skulls of Chionarchus 

 minor and C. crozettensis , especially as regards the shape 

 of the lacrymals (c/. text-figure 3). In passing, attention is 

 also drawn to the fact that in the genus Chionis the sagittal 

 ridge separating the supraorbital depression is single. In 

 Chionarchus it is double. 



Shufeldt (/. c), writing of these notches, says: " Their form 

 in Chionis agrees best with Hamatopus, but in Hamatopus the 

 foramina arc not entire, their lateral margins having given 



