506 ARKIV FÖR ZOOLOGI. BAND 1. 



in all Skuas. Whether a small first labial has disappeared 

 in these birds or not is for the present impossible to say 

 and thus also to decide whether the long labial jnst men- 

 tioned is the first or the second. There are also sometimes 

 traces hinting at a fusion of several labial plates although 

 this is uncertain. The »cere» of Stercorarius has the same 

 situation as the nasal tube of the Fulmarus and is no doubt 

 homologous with the same to its origin. 1 The internasal 

 tract is in both these birds covered by the nasal tnbe resp. 

 the cere. The rhamphotheca of the lower jaw of the Skua 

 exhibits as many pieces with almost the same situation as 

 in Fulmarus. The unpaired of the former is, however, more 

 strongly developed and extends further back on the rami of 

 the mandible and there is no indication as to its origin from 

 several fused plates as in Fulmarus (conf. above). The ex- 

 tension of this plate may be seen on the figure. Behind the 

 proximal end of the mentale the Skuas have just as Fulmarus 

 two quite distinct plates, the one, infralabial, bordering the 

 upper margin of the lower jaw proximally, the other, sub- 

 mandibular, below this one. In consequence of the great 

 development or especially backward extension of the mentale 

 these two plates appear to be sh orter (conf. however, below 

 about the condition in the young Larus) in the Skua than 

 in Fulmarus, but otherwise they are similarly arranged. 

 The infralabial is best defined, often all round, the sub- 

 mandibular is as a rule anteriorly quite fused with the 

 mentale but sometimes its anterior limit is quite conspicuous 

 as in the specimen of Stercorarius pomarinus which has been 

 the original to the above figure. In Larus the cere has dis- 

 appeared, but otherwise the bill resembles considerably that 

 of Stercorarius, and very often, especially in young although 

 fullgrown specimens of large species, but also in adult ones, 

 the sutures between the infralabial, submandibular and men- 

 tale are well conspicuous. In young Lari it appears as if 

 the submandibulars should extend further forward, reach the 

 symphysis and join there. In older specimens nothing defi- 

 nite can be seen concerning this. The labial of the upper 

 rhamphotheca is as a rule well defined in Sea-gulls. — In 

 the Terns (Sterna) the different elements of the rhamphotheca 

 are more fused together to form the pointed fish-spear of 

 1 There is sometimes an indication of a median division of the cere. 



