OLASSIFICATION OF BIRDS. 55? 
Crass VII.—AVES, 
Feathered Vertebrates ; jaws encased in horny beaks in existing forms : 
the fore-limbs forming wings; warm-blooded ; heart four-chambered ; 
lungs with accessory air-sacs; the bones dense, hollow ; oviparous ; eggs 
very large, covered by a calcareous shell. 
Sub-class 1. Saurure.—Tail as long as the body; head and fore limbs 
reptilian; with feathers, scales, and teeth. (Archeopteryx.) 
Sub-class 2. Odontornithes.*—Vertebre biconcave, or as usual; jaws 
slender, with teeth implanted in sockets or in grooves; meta- 
carpals co-ossified; sternum keeled or unkeeled ; wings well 
developed (Ichthyornis) or rudimentary (Hesperornis). 
Sub-class 3. Ratite.—Sternum smooth ; wingsrudimentary, (Struthio): 
Sub-class 4. Carinate.—Sternum keeled ; wings well developed. (Tur- 
dus, 
Laboratory Work.—The student should prepare a skeleton of a hen or 
any other bird, and compare it, and especially the skull and limbs, with 
those of a reptileand a mammal. In dissecting a pigeon or fowl, at- 
tention should be given to those points previously indicated in which 
birds diverge from reptiles on the one hand and mammals on the other. 
Crass [X.—Mammaria (Mammais). 
General Characters of Mammals.—In the mammals, which 
begin with the duck-bill, a creature in some respects re- 
minding us of the birds, and end with man, we observe, 
as compared with birds, an increased complexity of struc- 
ture ; and in the nature of the work done by the different 
organs, we may see a constant tendency to a development: 
of parts headward, so that the head becomes large in pro- 
portion to the body, the brain increases in size, and the fore~ 
limbs finally become hands, ministering to the intellectual! 
wants of the animal. Also, as we ascend the series, the body, 
from being horizontal, with limbs adapted for walking on all 
fours, becomes finally in the apes semi-erect, in man wholly so. 
The greatest step in advance over the reptiles and birds 
* It is doubtfulif this isa natural group. Ichthyornis was probably 
an archaic or generalized gull with teeth ; and the wingless Hesperornis 
was the ancestor of the grebes and loons.. (See also W. K. Parker.) 
