Systematic Position of the Sheath-bills. 149 



the orbital process is more slender and less truncated than 

 it is in the Skuas, in which it is relatively shorter, wider, 

 and with borders more parallel. 



Vomer. — In Chionis the vomer is hastate in shape; in 

 Hamatopus it is much wider, and is bifurcated at its extremity, 

 or, rather, is deeply notched. 



The vomers of the Skuas and Gulls are easily contrasted, 

 but are more like one another than they are like that of the 

 Sheath-bill. 



The Parasphenoidal Rostrum in the Sheath-bills presents 

 no facets for articulation with the pterygoids (no basi- 

 pterygoid processes). In Hcematopus, as is well known, 

 they are present. 



The Pterygoids in Hamatopus are short and typically 

 pluvialine. In the Sheath-bills they are distinctly peculiar, 

 being neither typically pluvialine nor larine. They are, 

 however, closer to the pluvialine type than the larine. The 

 pterygoids of the Skuas are certainly larine. 



Other Skeletal Features. 



As regards the rest of the axial and appendicular 

 skeleton, a few notes of a general nature seem to be 

 worth recording as throwing light on the affinities of the 

 Chionididse. 



Humerus. — This bone is distinctly pluvialine in its features. 

 The sub-trochanteric and tricipital fossae are not so distinct 

 or specialised as in the Laridse. Both the ridge separating 

 these fossae and the fossse themselves are, however, in the 

 Chionididae more sharply marked and accentuated than in 

 Charadrius and slightly differentiated. The humerus of the 

 Sheath-bills comes very close to that of Hamatopus and 

 Dromas in this respect. 



On the palmar aspect of the head of the humerus the 

 groove for the coraco-humeral muscle is not so deep or 

 conspicuous as in. the Laridse or Stercorariidee, but is more 

 sharply marked than in Charadrius or Hamatopus. In the 



