Systematic Position of the Sheath-bills. 139 



is somewhat rounded as in both the Gulls and Skuas — the 

 transverse diameter being longer than the sagittal. The 

 plane of the whole occipital area, including the plane of the 

 foramen magnum, makes a sharp angle with the basal plane 

 of the skull (a larine and Skua character). As a con- 

 sequence, these planes look distinctly backwards as well as 

 downwards. In Hamatopus the plane of the foramen 

 magnum looks directly downwards; and the same is nearly 

 true of the pluvialine genera Charadrius and Squatarola. 

 The supraoccipital ridge does not sweep forwards and 

 downwards to become merged in the inner border of the 

 paroccipital process, but ends abruptly in the middle of 

 either margin of the occipital foramen as two rather promi- 

 nent spinous processes, on the outer sides of which is a 

 distinct and well-defined groove (for the exit of the sinus 

 canal). These processes are not nearly so prominent in 

 Hcematopus. The arrangement in Charadrius is somewhat 

 different, the groove just mentioned being partly bridged. 



The lambdoidal ridge is not so sharply defined as in the 

 Laridffi. It is thicker and more osseous, and instead of being 

 continued outwards, forwards, and downwards as a sharply 

 defined ridge to run into the outer border of the iiaroccipital 

 process, it sweeps abruptly inwards as a thickened rounded 

 and more osseous ridge to terminate near the aforesaid 

 processes on either side of the foramen magnum. As a 

 result, the occipital area is divided in the Chionididae into 

 two distinct and hollowed surfaces separated by a prominent 

 ridge, and the identity of the supraoccipital and exoccipital 

 bones (which are separate entities in the embryo) is thereby 

 rendered more obvious (cf. figures). This appears to be a 

 pluvialine character, as it is to be noted in Charadrius; but 

 it is more exaggerated in the Sheath-bills. An interesting 

 fact to note is that it is to be observed in Stercornrius 

 crepidatus. It is indicated in S. parasiticus, and also in 

 Hcematopus, but is hardly present in Megalestris (antarctica), 

 which appears to be a more specialised stercorarine genus 

 than the rest. In the Gulls (Laridse) the separate identity 



