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ZOOLOGICAL EXERCISES. 
covered by small feathers which have an 
after-shaft ; wings with scale like feathers, 
and no distinct remiges; tail composed of 
narrow rigid feathers; anterior toes webbed : 
hind toe small and united to the tarsus; tarsi 
very short and broad, perforated by two 
longitudinal clefts; shaft of the humerus 
flattened. 
MAMMALIA.—Heart with two ventricles and two auricles : 
aortic arch single, on the left side; viviparous ; two 
occipital condyles; each ramus of the mandible is of 
one piece, and articulates with the squamosal bone of 
the skull; thorax divided from the abdomen by a 
diaphragm; body covered with hair; female with 
mammary glands. 
There are three sub-classes : 
Ornithodelphia. A large coracoid articulating 
with the sternum ; urinary organs, generative 
organs, and intestine opening into a common 
cloaca ; no placenta. 
Monotremata. Mammary glands without 
teats ; no teeth in the jaws. 
Didelphia. Coracoids merely processes of the 
scapula ; no placenta; vagina double; mam- 
mary glands with teats. 
Marsupialia. Marsupial bones attached to 
the pelvis ; female usually with a marsupium, 
or pouch. 
Moncdelphia. Coracoids merely processes of 
the scapula; a placenta developed from the 
allantois; vagina single; mammary glands 
with teats. 
