502 ARKIV FÖR ZOOLOGI. BAND 1. 



piece, but even otherwise and in other specimens a faint line 

 may be traced from the anterior margin of the nostril to- 

 wards the tip. In a young specimen of Colymbus adamsi 

 this line is very conspicuous and the same may be said 

 abont several specimens of Colymbus glacialis which I have 

 examined. This line according to my opinion separates a 

 median unpaired rostrale from a lateral labial on either side. 

 The nasal tract which basally divides the labial and rostral 

 elements is soft; the internasal is feathered. The lower 

 rhamphotheca is chiefly made up of an nnpaired and large 

 mentale, but proximally the gape seems to be bordered by 

 an infralabial which in some specimens is partly rather di- 

 stinct. It seems also probable that the portion below the 

 infralabial ought to be counted as corresponding to a sub- 

 mandibular, but it is fused whith the mentale. Even if the 



Fig. 11. The bill of Colymbus septentrionalis. 



reptilian homologies thus may be traced in the bill of the 

 Colymbi this is rather difficult because the organ in question 

 has in an analogous manner as in Anhingas and Herons 

 been specialised in its development to a »fish-spear» and this 

 has caused a fusion of the different elements in an indepen- 

 dent but parallel way in all three groups mentioned. 



The smooth bill of the Grebes makes also an analysis 

 of its constituting elements very difficult. There might, 

 however, in some specimens be seen an indication to a divi- 

 sion of the upper rhamphotheca in a median rostrale and a 

 lateral labial on either side. 1 Posteriorly the nostril forms 

 the dividing line between the median piece and the labial, 

 so that everything below and behind the nostril surely is to 

 be regarded as belonging to the labial. In front of the nostril 



1 In some specimens of Podicipes rubricollis I have seen such a divi- 

 sion very plainly. The bill of Podilymbus is somewhat less specialised, bnt 

 does not show the different elements more plainly than that of many typical 

 Grebes. 



