THE VOYAGE OP THE 'CHALLENGER.' 375 



genera, on the other hand (fig. 6), the aperture is more oval and distinctly 

 double, owing to the median septum (formed by the coalesced inner walls 

 of the narial tubes) being much less deeply, in a lateral view, excavated 

 anteriorly, and so appearing to a greater extent superficially. The other 

 Procellariinae repeat this form of nostril, though the septum becomes 

 much thicker, so that the nostrils open in them by two perfectly distinct 

 apertures (vide fig. 4, where the nostrils of Bulweria are shown). They 

 might thus be said to be ** platyrrhine," in opposition to the " catarrhine" 

 Oceanitidae and other genera already mentioned. It is in Bulweria and 

 Majaqueus, perhaps, that the nasal septum is broadest and most super- 

 ficial ; in Prion it is well developed; in the remaining genera it is less 

 near the external opening, but always quite evident. In the genus 

 Puffinus the septum is also broad, but the narial tubes are so obliquely 

 truncated that they hardly rise above the lateral outline of the beak ; 

 their openings are ovals, with their longer axis vertical or (Paffinus 

 obscurus) oblique inwards. 



In Pelecano'ides the nasal tubes are short and swollen externally ; the 

 septum is distinct, but not broad ; and the apertures, which are sinuated 

 ovals directed antero-posteriorly, look almost vertically upwards, their 

 lateral outline being nearly parallel with the axis of the upper jaw. 



In the Diomedeinae the nasal tubes are quite separate from each other, 

 lying just at the lower margin of the "culminicorn." They are usually 

 described as tubes with a distinct circular complete aperture ; but, on 

 looking at this carefully, there may be seen (vide fig. 1) in front and 



Fig. 1. 



Base of Beak of Diomcdea exulans, to show the form and position of the nostril. 



below this tubular opening a deep cavity leading bac-kwards and con- 

 tinuous behind, over the edge of the apparent outer boundary of the 

 aperture, with the general cavity of the tube, an infolding of the outer 

 wall of the latter forming the apparent outer wall of the tubular 

 aperture. 



