44 
ON THE CLASSIFICATION OF BIRDS. 
Glires, as also in pigs, anteaters, armadillos, moles, and 
shrews, all which groups are of the gliriform type, corre- 
sponding to the tenuirostral among the perchers. The 
eyes are defended and protected by two sorts of eyelids ; 
the first being tliose which are common to quadrupeds, 
and which are extenial : these, in the generality of birds, 
are clothed with small feathers, but are destituteof lashes: 
sometimes they are surrounded with a fleshy, naked skin, 
which is frequently so delicate as scarcely to be discerned, 
as in some of the American trogons, but this sort of 
ring in the turaccos {Corythaix 111.) is much broader, 
and begins to form a prominent dentated rim, the full 
development of which is seen in the genera PerspicUla 
{jig. 11.) and Prwnops. IVhat function this very sin- 
gular appendage is intended to perform, is altogether 
unknown ; but it is more prevalent in tenuirostral types 
than in any other ; and in the genus Zonterops it is re- 
presented by a circle of thick, glossy, dense feathers, of a 
snowy whiteness, which is more or less conspicuous in 
every species. It is chiefly in the family of cuckows 
{Cuculidtr), that we meet with eyebrows; they consist 
of a fringe, on the upper eyelid, of rigid, stiff bristles, 
placed singly, and sometimes much developed ; but at 
all times they are sufficiently long to overreach the 
ball of the eye. The genus Centropns, more especially, 
is thus distinguished ; the same structure, less deve- 
loped, may be seen in nearly all the Coccyzirue; hut in 
the European cuckoo, and its immediate allies, these 
eyelaslies are entirely wanting. There can be no doubt 
that this additional protection to the eyes is given to 
birds whose habits lead them into close thickets, and 
who explore dense foliage for their subsistence. ’ lEe 
know, at least, from personal observation, that such 
are the haunts of all the South American Cnccyzina;; 
and there is additional reason to believe in this con- 
jecture, from the peculiar hardness and rigidity of the 
feathers in Centnpus {jig. 20.), the shafts of which 
{a) are hard, glossy, and rigid, each ending in a point, 
so that, but for their webs (/;), they would become 
