16 THE VOYAGE OF H.M.S. CHALLENGER. 



Majaqueus, Puffinus, &e. The knee-gap may become so deep as to completely divide 

 the inferior tract into two parts below (e.g., Pelagodroma, Prion, and, according to 

 Nitzsch, Halobcena)} 



The hypopteruni is usually well-developed, with long feathers, and the humeral tracts 

 are very strong and broad. 



The contour-feathers always have an after-shaft, though in the Diomedeinse it is 

 extremely small, most so in Diomedea exulans where it is reduced to a short tuft, about 

 half an inch long, of five or six nearly simple, straight plumes. In the smaller Albatrosses 

 it is larger, and in the rest of the group, including Pelecanoldes, it is of good size. 



All the forms have their spaces as well as tracts covered by down-feathers, which 

 may become very long and close-set, especially in Pagodroma. 



The oil-gland is always large, globular, with its surface covered above at the base — 

 which is also partly covered by the termination of the uropygial band of the dorsal tract — 

 by scattered semi-plumes, and with a tubular mamilla, provided with a good tuft of 

 down-feathers. The tuft and gland are never absent. In the Oceanitidae and smaller forms 

 (Cymochorea, &c.) the tuft of feathers simply encircles the apex of the gland, but in the 

 larger ones it sends a median prolongation across it as well, so as to divide the surface 

 of the mamilla into two lateral parts, separated from each other by the median row of 

 feathers, and each with its opening or openings. The number of these varies in the 

 different forms of the group, as already indicated by Nitzsch (loc. cit., p. 144). Diomedea 

 exulans has about half a dozen small ones in each half, arranged in a crescent. Diomedea 

 brachyura and Thalassiarche have numerous small apertures opening into a single large 

 circular common opening. The Fulmars, except Aeipetes, have several apertures in each 

 half, as have Daption and Pagodroma, Ossifraga having as many as five. Majaqueus 

 has four; (Estrelata three. Aeipetes, Pelecanoldes, Bulweria, and the smaller Procellariidas, 

 as well as the Oceanitidae, have apparently only two pores, one in each half of the gland. 



The very young birds, I may remark, are, in all the species I have seen, covered with 

 a thick coating of fluffy grey down, which is pushed off as usual at the ends of the 

 contour-feathers when the latter appear. There are apparently no intermediate changes 

 of plumage, the first plumage of the young bird being similar to that of the adult," a 

 condition of things very unlike that in the Gulls (Laridae) with which the Tubinares have 

 so often been associated. Besides the long down on the tracts corresponding to the 

 future tracts of contour-feathers, the young birds have a shorter downy covering- 

 distributed pretty uniformly, as in the adults, over the intervening spaces, and between 

 the feathers of the tracts. 



1 Nitzsch lays some stress on the angle, whether acute or obtuse, made by the lumbar tracts at their junction with 

 the dorsal ; but the difference in the direction of the two parts is not, as seen in entire birds, so obvious as would be 

 judged from Nitzsch's figures (Joe. cit., pi. x. figs. 2, 3), which were probably made up from the examination of skins 

 only. The lumbar tracts, where the connecting rows of feathers are best developed, seem always to run outwards and 

 backwards from the dorsal tracts, as shown in his figure of Puffinus obscurus. 



2 Diomedea exulans may be an exception. 



