556 CHORD AT A. 



flying squirrels {Pierojnys), Anoinahirus^ the colugo or flying lemur 

 {Galeopitheciis) 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 patagiitm which stretches from 

 fore-limbs to hind-limbs and acts as a 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 serial. — The bats. These are the culminating group of the 

 aerial types. Here the serial habit becomes predominant. The patagial 

 surface becomes further extended, especially that part of it which can be 

 voluntarily moved in the neighbourhood of the fore-limb. 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 (Pteropodidae) 

 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 

 serial (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. 



