556 CHORDATA, 
flying squirrels (Pteromys), Anomalurus, the, colugo or flying lemur 
(Galopithecus) and, lastly, the bats. We may divide these into three 
groups :— 
1. The incidental group. —The phalangers, marsupial squirrels 
and true flying squirrels. All are arboreal and still adapted thereto. 
“Flying” is to them merely incidental, as is swimming to the first aquatic 
group. They have a thin fold of skin or patagiwm which stretches from 
fore-limbs to hind-limbs and acts asa parachute. In all, the tail is bushy 
and not only acts as a balancing organ in jumping but as a steering 
organ in flight. The spreading of this patagium is an easy addition to 
the long jumps from bough to bough performed by their ‘‘ non-flying ” 
allies. 
2. Transition type.—The colugo or flying lemur. In this the pata- 
gium extends further between the tail and the hind-limbs. The animal 
appears to have more direct means of steering itself, and flight is less 
‘incidental ” and more evenly balanced in the life of the animal with 
the arboreal habit. The limb-bones are long and slender to allow of a 
larger patagial surface. 
3. True zrial.—The bats. These are the culminating group of the 
zrial types. Here the zrial habit becomes predominant. The patagial 
surface becomes further extended, especially that part of it which can be 
voluntarily moved in the neighbourhood of the forelimb. The fore- 
limb and the digits are greatly elongated, forming axes for support of 
the patagium, the pectoral muscles are employed for movement and a 
keel on the sternum is the result. In one group of bats (Pteropodidz) 
two digits retain their claws and in the rest only one, the thumb. In 
the former the diet is still arboreal (fruits), but in the latter it is strictly 
zerial (insects). As, however, insects are not confined to the air we do 
not find a specially marked peculiarity in the teeth. The adaptations 
to flight are, therefore, mainly to be found in the locomotor organs. 
