138 ON THE CLASSIFICATION OF BIRDS. 
large as to contain an infinity of nerves, disposed like 
net-work, all of which lead immediately to the nostrils, 
and are protected externally hy a thin horny covering ; so 
that the biU, apparently so heavy, is in reality uncom- 
monly light, and is no inconvenience to the bird whatever. 
The interval between the toucans and the parrots is not, 
perhaps, so great as between the latter and the wood- 
peckers ; still it is sufficiently wide to make us believe that 
one, if not two, of the intervening types are wanting. 
The genera we restrict to four; Ramphastos, Pteroylossus, 
and Aiilacorynehu. 1 , being found in South America, 
while Sythrops is their representative in Austraha. 
(155.) The fifth and last family is composed of the 
Cuckows, — a numerous and diversified race, which is 
spread over all the temperate regions of the globe. Their 
principal distinction rests on the very sliort and slender 
make of their feet ; formed, indeed, much on the same 
model as those of the toucans, yet with one of the back 
toes so versatile that it can be occasionally brought 
forward, — a structure which we have already illus- 
trated*; while the habits of our European cuckow, 
which is a typical example of the whole, are known to al- 
most every one. The majority of the true cuckows build 
no nests : they fly rapidly, and have the nostrils perfectly 
round ; others, of the American genus Coccyzas, have 
sliort wings, and live almost entirely on small insects 
gleaned from the slender branches of lofty trees. The 
diversity of form observed in this family, renders it 
highly probable that all the genera of the two typical 
divisions are known, although the others have not vet 
been sufficiently studied. The following observations 
are, tlierefore, intended merely to show the apparent con- 
nection of this with the other scansorial families. 
(15(1.) The passage from the toucans to the euckows, 
seems to be marked by such genera as PhcenirophcBus or 
Saurothera, where the bill, as in the first, is either much 
larger and thicker than in the generality of cuckows, and 
is tlius assimilated in shape to that of the toucan, or, as in 
SauTOthera, where the edges become dentated. Crotophaga 
• Vol. I. p. 82 . 
