STERCORAKIIDJS STERCOKARIUS 451 



Livingstone, on p. 252 of his " Missionary Travels," as observed 

 by him on the Upper Zambesi near Libonta. 



Family IV. STERCORAHIID^E. 



Sternum with one notch on both sides posteriorily ; of the five 

 Garrodian thigh-muscles the accessory femorocaudal alone is absent ; 

 caeca long and well developed. 



Only one genus is here recognised, in the description of which 

 will be found the more prominent external characters. 



Genus I. STERCORARIUS. 



Type. 

 Stercorarius, Brisson, Orn. vi, p. 202 (1760) S. crepidatus. 



Bill stout and broad, shaped somewhat like that of a Gull, with 

 a terminal hook covering the tip of the lower mandible ; the basal 

 two-thirds covered with a horny cere which overlaps the nostrils, 

 so that the opening is reduced to a small, rounded or slit-like 

 aperture at the front end of the cere ; wings long and strong, the 

 first primary the longest ; tail long and round, but the two middle 

 feathers prolonged beyond the others, sometimes to a very con- 

 siderable extent ; tarsus stout with transverse scutes in front and 

 rounded scales laterally and posteriorly ; anterior toes very fully 

 webbed, posterior toe small and stumpy ; claws curved, sharp and 

 strong. 



Some seven species of Skua are generally recognised, four of 

 these are Arctic or north temperate, three Antarctic or south 

 temperate ; two of the northern forms and one of the southern 

 visit our shores. 



Key of the Species. 



A. Large ; wing over 16*0 ; central tail-feathers 



not projecting more than '5 inch S. ajitarcticus, p. 452. 



B. Intermediate ; wing 14'0 to 16'0 ; central tail- 



feathers broad, rounded at the ends, and 



projecting about 4*0 inches S. poma torhinus, p. 455. 



C. Smaller ; wing under 14*0 ; central tail-feathers 



pointed and tapering S. crepidatus, p. 453. 



