SKULL OF THE ZGITHOGNATHOUS BIRDS. 295 
Example 67. Skull of Swift (Cypselus apus), 1st summer. Family Cypselide. 
Group Tracheophone !. 
Habitat. Great Britain. 
We saw that this particular form of palate was present in birds lying at a great 
depth below the Passerine level ; there it was seen to be imperfect, although those birds 
possess every element ; it is arrested in the metamorphosis in those birds, the Hemipods 
(Turnix, Part I. pl. liv. Here, in the uppermost territory, in a small group, equal in 
genera and species only to one of the smaller Passerine families, we have a most distinct 
kind of bird with a perfectly Agithognathous palate. Therefore, if the morphology of 
the face is to count for much in the classification of birds, I do not see how Professor 
Husley’s “ Cypselomorphe” (P. Z. S. April 11, 1867, p. 468) can be retained. I have 
shown (Trans. Linn. Soc. ser. 2, Zool. vol. i. pp. 113-120) that two of the families placed 
by him in that group are schizognathous, this simple reptilian condition of the palate 
being retained by such high types as the Trochilide and the Caprimulgide. Feeling, as 
I do, that classification may be left to take care of itself, if the facts of the organization 
of the groups be made sure, I am not troubled to see that convenient little group break 
itself up into three lesser groups, each of equal value. 
Although the border of the Swifts falls to them close on that “top-land” of the 
Passerines where the Swallows congregate, yet are these conterminous groups only 
“second cousins,” and more alike in their habits and mode of dress than in their real 
nature. I mentioned that those Swift-shaped Flycatchers, the Swallows, come up to 
the true Sylviines in all that is normally Sylviine. Now a Swift, as to his skull and 
face, is merely an exaggerated Swallow, an wltra-hirundine bird—a caricature, as it 
were, of the true Passerine gaping birds. In the skeleton he comes close to the Hum- 
ming-birds ; in the huge disproportion in length of the arm to the hand, even the 
Swallow begins to be very Cypseline ; but the Swift and the Humming-bird are here at 
one. So, also, are they in the sternum and shoulder-girdle ; the Swift also has lost the 
“cxca coli,” and has not developed any intrinsic muscles to the syrinx (‘Shoulder-girdle 
and Sternum,’ plate xiii. p. 176; Macgillivray, Brit. Birds, vol. iii. pp. 606-626, pl. xxii. 
fig.5). These and many other characters that might be mentioned show that the Swift, 
although claiming to have arisen from the same essential stock and root as its Passerine 
relations, has, while failing to gain several of their most exquisite modifications, brought 
that kind of framework for which even the Swallow is remarkable to its uttermost 
degree of perfection 2. 
* The term “Tracheophone ” is applicable not only to those Coracomorph which have the syrinx imperfect, 
but also to the remainder of the Carinate birds. The title of these communications, ‘On the Agithognathe,” 
gives me great liberty; I am not bound to the Coracomorphe, although they yield me nearly all my materials ; 
but I search everywhere for this particular form of facial modification. 
* I think it is far from improbable that we owe that most exquisite creation, “Ariel,” to the poet’s familiarity 
