492 ARKIV FÖR ZOOLOGI. BAND 1. 



end of the nasal tube. The modified development of the pieces 

 of the lower jaw is somewhat more striking and at the first 

 look not so easily understood. The mentale of Puffinidce 

 appears to be not only confined to the lower side of the distal 

 end of the mandible, but to occupy the same space as the 

 mentale and first labial together occupy in Diomedea. It 

 is therefore probable that this, as it seems single, symphy- 

 sial shield which forms the härd tip of the mandible is of 

 a double origin and has been formed throngh the fusion of 

 an unpaired mentale and a pair of first infralabials situated 

 as described above for Diomedea. Behind this compound 

 shield the mandibular ramus is provided with two longi- 

 tudinal shields, one above and the other below. The upper 

 of these forms alone the mandibular margin. The homologies 

 of these two shields are most easily understood by comparing 

 them on one hand with the »Batitce» and on the other with 

 the Albatrosses. The result of this 

 comparison will undoubtedly be that 

 the upper one of these shields corres- 

 ponds to a second infralabial and the 

 lower one to a submandibular, and both 

 together to the large lateral shield of 

 Fig. 3. The rhamphotheca the lower rhamphotheca of the Alba- 

 of a young Fulmarus trosses as has been hinted above. The 

 glacialis. submandibulars are in Puffinidce not 



so fully separated at their anterior ends by the mentale as 

 in Diomedea, but meet and form a symphysial suture which 

 is especially well developed in Ossifraga, less so in Fulmarus. 

 From this analytic explanation it seems quite evident 

 that the lower rhamphotheca in all Tuoinares is compound 

 of the same elements although these are differently developed 

 in different forms. But no new elements are introduced and 

 none is excluded. The original elements are 7, viz. an un- 

 paired mentale and on either side a first and a second infra- 

 labial and below the latter a submandibular. These plates 

 are, however, not all present at the same time as free and 

 separate elements but a fusion of some has taken place, in 

 some birds between mentale and the first labials, in others 

 between the second labials and the submandibulars so that 

 the number of plates appears to be less than it really is. 

 The exteriör indicates therefore a difference which only is 



