8 2 Sclater on the Sy sterna Avium. 
1 6. Gaviae. 
In the ‘Nomenclator’ I have made the Gaviae to include the 
Petrels (Procellariidae) as well as the Gulls (Laridae). But I 
now think it better to restrict the term Gaviae to the latter group, 
the Longipennes of Nitzsch, which, not only as regards their 
osteology, but also in respect of their pterylosis, come very near 
to the Limicolae.* 
The Gaviae will therefore consist of the single family Laridae, 
while the Procellariidae will constitute the order 44 Tubinares ” 
(Nitzsch). The propriety of this separation is confirmed by 
what Prof. Garrod has stated (P. Z. S. 1879, p. 37) as to the 
form of the nasal bone in these two groups. 
17. Pygopodes, and 18. Impennes. 
The Pygopodes of Illiger combine the two families Colymbidae 
and Alcidae, which are also closely allied pterylographically. 
They seem to form a natural transition between the Gaviae and 
the Impennes. Nitzsch (Pterylogr. p. 15 1) has associated them 
with the latter group ; but the Penguins are very distinct not only 
in their osteology, but also in their pterylosis, as admitted by 
Nitzsch himself, and have full claims to constitute an order 
per se. 
19. Crypturi. 
Under this term (of Illiger) I have placed in the 4 Nomenclator’ 
the Tinamidae, which, as Mr. Parker has shown (Trans. Zool. 
Soc. v. p. 149), have a completely struthious palate, and in other 
respects come at the bottom of the series, and are nearest of all 
Carinate birds to the Ratitae. In so doing I make, of course, no 
claim to originality, but have simply followed Prof. Huxley, who 
first located the Tinamous in their position under the title * 4 Dro- 
maeognathae.”t 
20. Apteryges, and 21. Struthiones. 
In the table given in the 4 Nomenclator’ (p. iv) I have recog- 
nized only two orders of Ratite birds — Apteryges and Struthiones. 
But there is no doubt, I think, that, the Casualties have full claim 
* Cf. Nitzsch, Pterylogr. p. 141 ; and Huxley, P, Z. S. 1867, p. 458. 
f P. Z. S. 1867, p. 125. 
