CHIROPTERA 37 



typical for British vespertilionid bats. The Horseshoes differ 

 from them in certain important respects, and these have been 

 noted under each species. The period of gestation is longer, 

 extending to ten or twelve weeks ; the young at birth have the 

 upper surface covered with down ; and they hold on to their 

 dams by means of a special pair of false nipples situated in the 

 ingfuinal resfion. 



Differences of habit will also no doubt be met with in other 

 vespertilionid species. For instance, the number of young at a 

 birth is always restricted to one in the Mouse-ear, as it appears 

 to be also in every British species of this genus, as well as 

 in the Serotine and the Long-eared.^ The members of the 

 genera Nyctahts and Pipistrellus, however, present a curious 

 problem, since, although the occurrence of two young at a 

 birth is almost unknown in Britain, continental naturalists 

 frequently credit them with that number, although not at every 

 birth. On this point they are quite definite, and there is nothing 

 inherently improbable in the statement, since in many exotic 

 bats' twins or more are usual, and these are sometimes carried 

 about by the mother even when they have reached about two- 

 thirds of her size.^ Confirmation in the case of the genus 

 Pipistrellus comes from America, where Mr Vernon Bailey^ 

 finds that the Little Canyon Bat^ may have either one or two 

 embryos. The facts being as above stated — and, although 

 further investigation is desirable, there can, I think, be no reason 

 to doubt them — we have here a very interesting case of varia- 

 tion in fertility according to locality. My friend Mr Moffat 

 points out^ that the lower degree of fertility occurs on 

 the outer limits of the ranges of these bats. He compares it 

 with somewhat similar facts in British and Irish birds, which 



' For Barbastella alone I have no information. 



- See W. H. Hudson for Molossus bonariensis in The Naturalist in La Plata, 10 1- 

 104 : Chapman and Hall, Limited, London, 1892 The Red Bat of N. America has 

 four mammae, and one weighing 1 1 grammes has been taken alive nursing four 

 young, weighing collectively 127 grammes (M. W. Lyon, jun., Proc. U.S. Nat. Miis., 

 xxvi., 425-426, 1903). 



^ North American Fauna, No. xxv., 210, 1905. 



^ P. hesperus (H. Allen) ; P. abramus of Java has also been found with two 

 embryos (see G. M. Allen, Bull. Mus. Comparative Zool., Harvard College, Hi., 3, 

 July 1908, 45) ; and P. subflavus of N. America with three (Hahn, op. cit., 162). 



'" "The Problems of an Island Fauna," Irish Naturalist, 1907, 141. 



