EXTERNAL ANATOMY. — MOBILE CRESTS. 
39 
to these we add the Phibalura cristata*, one of the 
swallow chatterers; the OxyThynchus cristatus^ , among 
the climbing creepers ; and two species of Tiaris, among 
the tanagers — we are not aware of any other birds, 
independent of the American flycatchers, whereon this 
peculiar sort of crest is developed. 
( 47 .) The third class of crests differ from the two 
former in not being mobile ; that is to say, they are of 
such a structure as always to remain, more or less, fixed 
and elevated in the same position. They are formed by 
the feathers on each side of the head being disposed as 
if they had been raised up, and then compressed to- 
gether, so as to leave a ridge or keel rising up along the 
centre of the skull, precisely similar to the elevated 
ridge of a helmet ; or, to express this structure more 
technically, these crests are vertically compressed, and 
carinated. There are, comparatively, so few exam- 
ples of this structure, that we should not expect to find 
it much diversified ; and yet it appears under many 
curious modifications. The first, or incipient indication 
of this form, is seen in 
those birds which, like 
the satin grakles {Pti- 
lonoryv chits, fey. 1 7-) = 
many of the true gra- 
kles iLamprotorninte'), 
have the small frontal 
feathers closely pressed 
round the base of the nostrils and upper mandible, and 
very slightly raised just above the front : tbis, however, 
cannot be caUed a crest, but is rather the commencement 
of one. In several of the swallow shrikes (^Edolinot'), 
nature advances another step ; for here we see some of 
the frontal feathers not only more elevated than the 
others, but even slightly elongated ; next we find them 
curled backwards, so that the drongup of Le Vaillant+, 
* Zool. Illust. i. pi. 13. . , 
Cres ted sharpbjll, Zool. Illust. i. pi- 49. 
t Ois. d’Afrique, pi. 170. 
D 4 
