VERTEBRATA: MAMMALIA. 761 



great Trogontherium (fig. 441) is Pliocene and Post-pliocene, 

 and the equally gigantic Castoroides of North America is also 

 found in the Post-pliocene deposits. 



CHAPTER LXXVII. 

 CHEIROPTERA. 



ORDER XIV. CHEIROPTERA.* This order is undoubtedly 

 "the most distinctly circumscribed and natural group" in the 

 whole class of the Mammalia. The most obvious peculiarity 

 of the Bats is the modification of the hand for the purpose of 

 supporting a flying-membrane ; but with this are correlated 

 other structural characters of importance. 



The Cheiroptera are essentially characterised by the fact that 

 the anterior limbs are longer than the posterior, the digits of the 

 fore-limb, with the exception of the pollex, being enormously elon- 

 gated (fig. 442). These elongated fingers are united by an ex- 

 panded membrane or " patagium," which is also extended between 

 the fore and hind limbs and the sides of the body, and in many 

 cases passes also between the hind-limbs and the tail. The pata- 

 gium thus formed is naked, or nearly so, on both sides, and it 

 serves for flight. Of the fingers of the hand, the pollex, and 

 sometimes the next finger as well, is unguiculate, or furnished 

 with a claw ; but the other digits are destitute of nails. In the 

 hind-limbs all the toes are unguiculate, and the hallux is not in 

 any respect different from the other digits. Well-developed clavicles 

 are always present, and the radius has no power of rotation upon 

 the ulna. The mammary glands are two in number, and are 

 placed upon the chest. There are teeth of three kinds, and the 

 canines are always well developed. The molars are tiiberculate 

 or grooved in the frugivorous forms, and cuspidate in the insectiv- 

 orous species. The ulna is rudimentary. The bones are not pneu- 

 matic. The testes are abdominal except during the breeding 

 season. The stomach is complex and the intestine long in 

 the fruit-eating Bats ; but the reverse of this obtains amongst 

 the insectivorous forms. The Cheiroptera are cosmopolitan 

 in their distribution, and the oldest known species is from the 

 Eocene rocks. 



* The Cheiropfera'-WQrz placed by Linnaeus in his order Primates, which 

 contained also the Lemurs, the Apes, and Man. 



