394 
NOTES. 
vertebrae vary in form : in the lowest they are biconcave, like those of Fishes; 
in Salamanders they are opisthocoelous : in the Frogs and Toads they are 
usually procoelous. 
162 Salamanders are often taken for Lizards, but differ in having gills in 
early life and a naked skin. The Proteus and Siren resemble a tadpole ar¬ 
rested in its development. 
163 The Surinam Toad has no tongue. 
164 The posterior pair of limbs is sometimes represented by a pair of small 
bones ; and the Boas and Pythons show traces of external hind-limbs. 
165 There are some notable exceptions. The Slow-worm is legless, and 
the Chameleon has a soft skin, with minute scales. • 
166 According to Owen ; but Huxley insists that the plastron belongs to the 
exoskeleton. 
167 Knees always bend forward, and heels always bend backward. 
168 We cannot claim that this airy skeleton is necessary for flight. The 
bones of the Bat are free from air, yet it is able to keep longer on the wing 
than the Sparrow. The common Fowl has a hollow humerus; while some 
Birds of long flight, as the Snipe and Curlew, have airless bones. 
169 The fossil Archaeopteryx , a lizard-like Bird, is placed in a separate 
division, Saururce. Birds have also been divided according to their degree 
of development at birth into (1) Hesthogenous , as Fowls, Ostriches, Plovers, 
Snipes, Rails, Divers, and Ducks, whose chick is hatched completely clothed, 
has perfect senses, runs about, and feeds itself. When full grown, it uses its 
first opportunity to settle on land or water, not on trees; the male is po¬ 
lygamous and pugnacious; the female makes little or no nest; and neither 
sex sings. This group is of the best use to man, and approaches more nearly 
to Mammals, the habitual use of the legs and preference for land or water 
degrading it as a Bird and raising it in the list of animals ; (2) Gymnogenous , 
as Gulls, Pelicans, Birds of Prey, Herons, Sparrows, Woodpeckers, and 
Pigeons, whose chick comes helpless, blind, and naked; it can neither walk 
nor feed itself, but gapes for food; the adult is monogamous, and builds 
elaborate nests in trees and perches; many sing; all are habitual flyers. 
These are birds par excellence, gifted with higher intelligence than the others, 
and are never domesticated for food. 
170 Hopping is characteristic of and confined to the Perchers; but many of 
them, as the Meadow-lark, Blackbird, and Crow, walk. 
171 This order is artificial. But it is better to retain it until ornithologists 
agree upon some natural arrangement. The classification of birds is taken 
from Coues’s “ Key to North American Birds,” as being the work on orni¬ 
thology in most general use. 
172 The whales are hairy during foetal life only. 
178 The Manatee has 6; Hoffmann’s Sloth 6; and two species of three-toed 
Sloth have respectively 8 and 9. 
174 As in the Whale, Porpoise, Seal, and Mole. Teeth are wanting in the 
Whalebone Whales, Ant-eaters, Manis, and Echidna. 
175 The Monotremes resemble Marsupials in having marsupial bones, but 
have no pouch. They differ from all other Mammals in having no distinct 
nipples. 
