782 MAMMALIA. 
is loose in the males, unformed in the females. The knee is 
turned backwards like the elbow; the ankle has a cartil- 
aginous prolongation or calcar, which supports the fold of 
skin between limb and tail; the five toes are clawed. 
The vertebral column is short; there is little mobility 
between the vertebrz ; neural spines are absent behind the 
third cervical, except in Pteropide ; the caudal vertebrze are 
very simple. The ribs are usually flat. The maximum 
dentition is 353 ; the milk-teeth are very different from the 
permanent set. All the bones are slender, and the long 
‘bones have relatively large medullary canals. 
The cerebral hemispheres are smooth, or with few con- 
-volutions, and leave the cerebellum uncovered. The spinal 
‘cord is at first very broad, but narrows rapidly behind the 
neck. The sense of touch is remarkably developed in the 
hot skin of the wing, the large mobile external ears, the 
whisker hairs of the snout, and in the strange plaited “ nose 
leaves” around the nostrils. Even when deprived of sight, 
‘hearing, and smell, bats will fly about in a room without 
‘striking numerous wires stretched across it. The stomach 
is usually simple, but there is a long pyloric diverticulum, 
filled with coagulated blood, in the blood-sucking Desmodus. 
The whole gut is very short in insectivorous forms. There 
iis never more than a very short czecum. 
The temperature of the body is high. The testes are 
abdominal or inguinal; the penis is pendent. The uterus 
is simple, bicornuate, or duplex. There is usually but one 
offspring at a time, and there are never more than two. 
The mamme are two in number, thoracic, generally post- 
axillary in position. As in Insectivora and Rodentia, the 
yolk-sac forms a provisional placenta, and the allantoic 
placenta is discoidal and deciduate. “ What looks like 
menstrual flux has been noticed in some bats. In most 
European bats sexual union occurs in autumn, but the 
sperms are simply stored in the uterus, for ovulation and 
fertilisation do not take place till spring—after the winter 
sleep. In exceptional cases, especially in young forms 
which were not mature in autumn, pairing occurs in spring. 
Fossil Chiroptera occur in Upper Eocene strata, but are 
quite like the modern forms. 
