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 vertebrae; neural spines are absent behind the 

 third cervical, except in Pteropidae ; the caudal vertebrae are 

 very simple. The ribs are usually flat. The maximum 

 dentition is ~~ ; 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 

 is never more than a very short caecum. 



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 mammae 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. 



