638 CHORDATA. 



tail, includes the lower extremities to the foot, and extends thence 

 to the fingers, leaving the thumb free. Fingers 2-5 are enormously 

 elongated and support the membrane. Since flight requires 

 strong muscles, the sternum develops a small keel, recalling that 

 of birds, for the attachment of the large pectoral muscle. In con- 

 nexion with the flying powers the clavicle is strong. The patagium 

 is the seat of a very acute tactile sense, by means of which blinded 

 bats can fly among all kinds of obstacles without disturbing them. 

 The enormous ear conchs and a noticeable nose leaf, widely dis- 

 tributed through the group, also have marked tactile powers. In 

 the pectoral position of the mammary glands and in the discoidal 

 placenta these animals resemble the primates. In temperate 

 regions bats hibernate during the winter. The dentition is vari- 

 able, often fllf . Fossils occur in the eocene. 



Sub Order I. MICROCHIROPTERA, with insectivorous dentition, 

 only the thumb of the fore limbs clawed. VESPERTILIONID^, tail long, no 

 nose leaf ; Vesperugo* Atalapha* PHYLLOSTOMID^E, with nose leaf, trop- 

 ical America ; Desmodus, the blood-sucking or vampyre bat. 



Sub Order II. MACHROCHI ROPIER A (Frugivora), with smooth- 

 crowned molars, claws on thumb and first two fingers. Includes the flying 

 foxes, Pteropus, of the East Indies. 



Order IV. Rodentia. 



The rodents unite great similarity in appearance with a char- 

 acteristic dentition. The canines are absent, and the molars are 

 separated by a large gap (diastema) from the incisors (fig. 663). 



The latter are strong, chisel-like, 

 have persistent pulps and grow at 

 the lower end as they are worn 

 away at the cutting edge. Since 

 only the front surface has enamel, 

 wear keeps them constantly sharp. 

 Usually there is but a single in- 

 cisor, and only in the Duplici- 

 dentata is a second present in the 

 upper jaw. The molars are cus- 

 FIG. 663. skull of porcupine. (From pidate or have enamel folds and 



Schmarda.) /, frontal; im, premaxil- * 



lary; k, temporal fossa continuous in frequently Continue to ffl'OW 

 front with orbit; o, infraorbital fora- J . 



men, enormous on account of the por- throughout llie. Their number IS 

 tion of the masseter muscle which , , , , . , _ 



passes through it. frequently reduced, the formulae 



varying between f--f | and -J-J-J-f. Many species have an inflected 

 angle of the jaw like that of marsupials. The infraorbital canal 

 is a striking feature in Muridae and Hystricidae (fig. 663, 0), a 



