EODENTIA, CHEIROPTERA, AND INSECTIVORA. 411 



Fam. 12. Sciuridcc. — This is the last family of Eodents 

 which calls for any special mention, and it comprises the 

 true Squirrels, the Flying Squirrels, and the Marmots. 



The members of this family are distinguished by their 

 pointed or compressed incisors and their tubercular molars, 

 the upper jaw having five of the latter on each side, whilst 

 the lower jaw has only four. The genus Sciums, comprising 

 the true Squirrels, is represented from the Eocene Tertiary 

 upwards, but none of the fossil forms are of special interest. 

 SciuraviLS and Paramys appear to represent the true Squirrels 

 in the Eocene of North America, as does Iscliyromys in the 

 Miocene Tertiary of the same region ; while the AUomys of 

 the American Miocene may perhaps be related to the Flying 

 Squirrels. The earliest form representative of the existing 

 Marmots (Arctomys) seems to be the Flesiardomys of the 

 Eocene of France, which has relations to the Squirrels. 

 Arctomys itself^ does not appear till the Miocene is reached; 

 and there are several Post - Tertiary forms. Lastly, the 

 Pouched Marmots (Spermophilus) appear for the first time 

 in the Miocene ; and Britain possessed two species in times 

 posterior to the Glacial period. 



Order XIV. Cheiroptera.^ — This order is, from one point 

 of view, " the most distinctly circumscribed and natural 

 group " in the whole class of the Mammalia. In many 

 respects, however, it would be advantageous to regard the 

 Cheiroptera as a sub-order of the next order (namely, the 

 Insedivora) specially modified to lead an aerial life ; just 

 as the Pinnigrada are regarded as a mere section of the 

 Carnivora specially modified to suit an aquatic life. 



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 enor- 

 mously elongated (fig. 685). These elongated fingers are united 

 by an expanded membrane or " patagiiim" which is also ex- 

 tended 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 patagium thus formed is naked, or nearly so, on 



^ The Cheiroptera were placed by Linnaeus in his order Primates, which con- 

 tained also the Lemurs, the Apes, and Man. 



