170 NATURAL HISTORY. 
teresting portion of the Grouse family. They live in the 
far north in America and Europe. Their legs, and even 
the feet, are covered with hair-like feathers. Their plu- 
mage, like the fur of the Ermine and some other quadru- 
peds, changes, as winter comes on, from a rich, almost 
tortoise-shell color, to a pure white. The trade in Ptar- 
migans in the north of Europe is very extensive. The 
captured birds are kept in a frozen state for the dealers 
who come for them. 
278. The Sheath-bills are a comparatively small family, 
found chiefly in South America. Their nostrils are sur- 
rounded by a kind of sheath, and their plumage is snowy 
white. The Tinamous family, also a small one,is found 
in the same country, where they seem to occupy the 
same place that the Partridges and Quails do in other 
countries. The family of Greatfoots is peculiar to Aus- 
tralia and the adjacent islands. One of them is called 
the Brush Turkey, from its resemblance in general form 
to the common Turkey. It lives in the thick brushwood 
of Australia. This and another bird, the Mound-making 
Megapode (Greatfoot), are famous for making the mounds 
spoken of in § 205. This latter bird deposits its eggs 
some five or six feet deep in its mound, and then covers 
them up. Its mounds are very large. One of them was 
found to be fifteen feet high and sixty feet in cireumfer- 
ence. They were at first supposed to be the tombs of 
the aborigines. ) 
279. We come now to the order Cursores, or Runners 
—the Ostriches and their allies) We commonly think 
of birds as being, of course, capable of flight, but here we 
have a class of birds which are wholly terrestrial. Near- 
ly all of them have wings, but all that their wings can do 
is to assist them in running. Their wings being small, 
the muscles which move them are small also. Accord- 
ingly, the breast-bone is entirely destitute of the project- 
ing keel (§ 199) which it has in other birds, this being 
needed only for the attachment of large muscles. In the 
