60 PROCELLARIIFORMES CHAP. 
horny sheath is separated by grooves into more or less distinct 
plates, and the mandible may also be grooved, as in Phoebetria ; 
while Prion is especially remarkable for the curious fringe of trans- 
verse lamellae on the margins of the broad maxilla, which recall 
those of the Duck tribe, traces of the same being exhibited by 
Ossifraga, Fulmarus, Daption, and Halobaena. The most striking 
peculiarity, however, is the tubular structure of the impervious nos- 
trils, which trenchantly divides the Petrels from all other Birds ; 
these tubes are far apart in the Diomedeinae, and lie laterally 
towards the back of the culmen; in the remaining groups they 
are fused together and are situated dorsally. In the Oceanitinae 
the single aperture looks forwards and upwards, but in the 
Procellariinae the septum is produced to the front, showing 
clearly the double nature of the formation; in Pelecanoides 
again the distinct openings are almost vertical, an arrangement as 
well adapted to the diving habits as are the long sternum and the 
compressed wing-bones. The rows of retroverted spines found on 
the palatal membrane in most of the family no doubt aid in the 
retention of slippery prey, as do the lamellae in Prion. The 
lower portion of the tibia is bare; the metatarsus varies in 
length and stoutness according to the species, though often 
decidedly slender, and is much compressed in Puffinus and its 
nearest allies. It is usually covered with hexagonal scales, but 
Oceanites and Cymodroma show but one long anterior scute 
(ocrea), while Garrodia and Pelagodroma have a series of 
oblique plates instead. The hallux is absent in Pelecanoides, and 
consists of only one phalanx elsewhere, being quite rudimentary in 
the Diomedeinae ; it is slightly above the level of the anterior 
toes, which are connected by large webs. The claws are, as a 
rule, sharp, curved, and compressed, but are blunt and much 
flattened in Pelagodroma, Pealea, and Cymodroma, showing a 
similar tendency in others of the Oceanitinae. The wings are 
normally long, and are very narrow and pointed in the Diome- 
deinae, where the expanse is vast, but in Pelecanoides they are 
decidedly short: the primaries are eleven in number; the 
secondaries are ten or less in the Oceanitinae, thirteen or more 
in the remaining forms, and amount to more than thirty in some 
of the Diomedeinae. The tail is rarely long, as in Phoebetria, and 
may be even, rounded, graduated, or emarginated; the above species, 
Bulweria, and some forms of Pujginus have it wedge-shaped, while 
