144 ■ Mr. P. R. Lowe on the 



•way converting them into deep notches." As will be seen 

 by a reference to the figures, the notches in the genus 

 Chionis may or may not be converted into complete foramina, 

 and the same applies to the genus Chionarchus, so that it 

 does not appear to be a question of the lateral margins 

 having " given way " in Hcematopus, but rather that they 

 are not so specialised, or do not have the same tendency 

 to specialise, as in the Sheath-bills (or Skuas). In my 

 opinion, however, the form and general configuration of 

 the notches and supraorbital depressions agree best with 

 Stercorarius, but one might almost as well have compared 

 them with any of the aberrant Plover-like forms already 

 mentioned, and not only with these but with Squatarola, in 

 which Ave can observe a more generalised but still funda- 

 mentally similar condition appertaining to these supraorbital 

 grooves and the notches under discussion. 



As regards the lacrymals, the orbital portions of these in 

 the Chiouididae present considerable variation both in form, 

 structure, and size, corresponding not only to generic differ- 

 ences but also to intra-generic variations. They are quite 

 peculiar to the group, but there is a skull in the Natural 

 History Museum of uncertain locality, and labelled " C/iionis 

 alba, bought of Mr. Thompson," in which the lacrymals 

 appear to be of a more generalised form and to come rather 

 close to those o^ Hamatopus [cf. text-figure 3 C). 



In Chionis the lacrymals are distinctly pneumatic, and 

 there is a varying amount of hyperostosis. In Chionarchus 

 the lacrymals are flat plate-like structures. The descending 

 processes of the lacrymals in the Sheath-bills are somewhat 

 abortive, but pluvialine in form and structure. It may be 

 noted here that these processes in the Skuas and Gulls are 

 sharply contrasted. In the Gulls (Laridae) the descending 

 process of the lacrymal makes a very sharp angle with the 

 orbital process, and approaches the middle or lower portion 

 of the outer edge of the antorbital plate from a long way 

 distad of it. In the Skuas the angle made is a right angle, 

 and the descending process passes perpendicularly to the 

 upper angle of the outer edge of the antorbital plate. In 



