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



In the Procellariidse, except Pelecanoulcs, 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 in all the species 

 in which I have examined into this point — remain partially distinct, being 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. 1 



There are always large supra-orbital glands, which occupy depressions excavated for 

 them in the top of the skull (vide PI. VI. fig. 3), and open by a small duct into the 

 nasal cavities. Similar glands occur in many birds, notably the Penguins, Colymbidse, 

 Auks. Gulls, and many others. 2 



As in all other Ciconiiform birds, there is no true penis developed. 



5. Trachea and Vocal Organs. 



The trachea in all Tubiuares 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 — Fidmarus, 

 T/udassceca, 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 I 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 Gallinas, 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 Procellariidse. 



2 Cf. Nitzsch's article, " Ueber die Nasendriise der Vogel," Meckel's Archiv, 1820, pp. 234-269. 



