THE VOYAGE OF THE ' CHALLENGER.' 397 



interelavicular air-cell, but at least in all the species in which I have ex- 

 amined into this point remain partially distinct, being separated for th j 

 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 Peleca- 

 noides the ordinary structure prevails *. 



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

 excavated for them in the top of the skull (vide PI. XXII. fig. 3), and open 

 by a small duct into the nasal cavities. Similar glands occur in many 

 birds, notably the Penguins, Colymbidae, Auks, Gulls, and many others t. 



As in all other Ciconiiforoi 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, Thalassceca, 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 Gallinae, 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 them- 

 selves, 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 

 muse-les (of which of course there are only a pair) to a particular bronchial 

 semi ring is constant, thereby affording a landmark by which the con- 

 tiguous rings on both sides can at once be assigned to their proper posi- 

 tion. 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 



* 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. 



t Cf. Nitzsch's article, " Ueber die Nasendruse der Vo'gel," Meckel's Archiv, 1820, 

 pp. 234-269. 



