The ducks and geese form alone the order _Lamellirostres_, in which is included the curious bird _Palamedea_, which is a goose adapted to live in trees in harmony with its South American forest habitat.
The rails and coots go with the bustards and cranes to constitute the order _Alectorides_. Similarly the auks, divers, puffins, terns, and grebes, noddies, and guillemots may be associated together in one order--the order _Pygopodes_. The gulls and petrels form another association--the order _Gaviæ_; while the plovers, snipes, curlews, peewits, turnstones, &c., constitute the order _Limicolæ_. The order _Heridiones_ includes the herons, the bitterns, the storks, spoonbill, ibis, &c.
All the foregoing birds have a multitude of points in common; indeed, so close is the similarity of their structure that their subdivision into orders is a matter of much difficulty and dispute. They are collectively spoken of as the _Carinatæ_, from the keeled form of their breast-bone.
Widely apart from them stands another group made up almost entirely of large birds, which agree not only in having no power of flight, but also in certain significant structural characters, amongst which may be mentioned the absence of a keel on the breast-bone.
This latter group is sometimes spoken of as the order _Struthiones_ from the ostrich (_Struthio_), which is its typical form. Sometimes these keelless birds are called _Ratitæ_. Besides the ostrich, the rhea, cassowary, and emeu are included within the group; also the small and nocturnal _Apteryx_ of New Zealand and those giants of featherdom, the huge species of dinornis, all also of New Zealand and all now extinct.
With this our list of birds might close, but for a bird which anciently existed in Europe so strangely different from all modern kinds, that it must certainly be here adverted to. This bird is the _Archeopteryx_, found in fossil in the Solenhofen States.
The class Aves, like the class Mammalia, consists of animals with hot blood, but all birds have feathers and a number of other peculiarities of structure, as will appear later.
The next class to be adverted to is the class which includes all reptiles properly so-called--the class _Reptilia_.