460 ORDERS OF MAMMALIA. 



Remains of a species of Jerboa (Dipus) have been discovered 

 in Miocene deposits in France, but they are of no special im- 

 portance. 



Fam. 7. Myoxidce. The members of this family are com- 

 monly known as Dormice, and they are often included in the 

 following family of the Squirrels and Marmots. 



They resemble the Squirrels in most respects, but they have 

 only four molars on each side of the upper jaw, whereas the 

 latter possess five. Two species of Myoxus have been detected 

 in the Upper Eocene (Gypseous series of Montmartre), and a 

 third species has been determined from beds of Miocene age. 

 Several species have been detected in Post-Tertiary deposits, 

 of which the most remarkable is the comparatively gigantic 

 Myoxus Melitensis of the Maltese Post-Pliocene. 



Fam. 8. Sciuridcz. This is the last family of Rodents which 

 calls for any special mention, and it comprises the true Squir- 

 rels, 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 Sciurus, comprising the true Squir- 

 rels, is represented from the Eocene Tertiary upwards, but 

 none of the fossil forms are of special interest. The genus 

 Arctomys, comprising the Marmots, is represented in the Plio- 

 cene by a single species, and by several forms in the Post- 

 Tertiary. The Pouched Marmots (Spermophilus), lastly, ap- 

 pear for the first time in the Miocene ; and Britain possessed 

 two species in times posterior to the Glacial period. 



ORDER XL CHEIROPTERA. This order is undoubtedly " 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 Insectivord), 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. 373). These elongated fingers are 

 united by an expanded 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 patagium thus formed is naked, or 

 nearly so, on both sides, and it serves for flight. Of the fingers 



