32 THE VOYAGE OF H.M.S. CHALLENGER. 
In the Procellariide, except Pelecanoides, the two most anterior air-cells, which lie 
between the rami of the furcula at the entrance to the thorax, are not, as is usually the case 
in birds, fused together to form an interclavicular air-cell, but—at least m all the species 
in which I have examined into this pomt—remain partially distinct, beg separated for 
the greater part of their length by a median septum formed by the coalescence of their 
internal walls—and double in consequence—but imperfect behind in the middle line, 
so that there is here a free communication between the two cells over the trachea. In 
the Oceanitidse and Pelecanoides the ordinary structure prevails.! 
There are always large supra-orbital glands, which occupy depressions excavated for 
them in the top of the skull (wide Pl. VI. fig. 3), and open by a small duct into the 
nasal cavities. Similar glands occur in many birds, notably the Penguins, Colymbidee, 
Auks, Gulls, and many others.’ 
As in all other Ciconuform birds, there is no true penis developed. 
5. TRACHEA AND VocAL ORGANS. 
The trachea in all Tubinares is a straight, simple tube, never convoluted in any way, 
and with the normal structure of this organ in birds. In some of the genera—Fulmarus, 
Thalasseca, Aeipetes, and Ossifraga—it is divided, as will be described in detail further 
on, to a greater or less extent by a median longitudinal septum, as in the Penguins alone 
of other birds so far as | know. The trachea has the ordinary long lateral muscle on each 
side, as well as a pair of well-developed sterno-tracheales, these arising from the costal 
processes of the sternum, as in so very many birds. 
The constitution of the syrinx, or lower larynx, differs very considerably in the 
different genera and groups of the Tubinares as regards the number and modifications of 
form of the various tracheal or bronchial rings that enter into its composition. When 
as, e.g., in the Galline, the syrinx has no intrinsic muscles, the only guides for 
determination of the exact rings forming the syrinx are the variations in form of 
the rings themselves, according as to whether they are tracheal or bronchial, and the 
facts elucidated by a comparative study of these parts in a series of genera. Such 
a study of the syrinx in the Tubinares has made it evident to me that in this 
group at least the attachment of the intrinsic syringeal muscles (of which of course 
there are only a pair) to a particular bronchial semi-ring is constant, thereby affording 
a landmark by which the contiguous rings on both sides can at once be assigned to their 
proper position. The semi-ring that bears the muscle in the Tubinares is the fifth, the 
four bronchial rings (or semi-rings) above it, as well as a less or greater number of the 
1 In one of the three specimens of Oceanites examined, there appeared to be a division of the interclavicular air-cell 
into two, as in the Procellariide. 
2 Of. Nitzsch’s article, “ Ueber die Nasendriise der Vogel,” Meckel’s Archiv, 1820, pp. 284-269. 
