220 MISS DOROTHEA BATE ON A NEW GENUS 



Summary and Conclusions. 



The above description may be very briefly sumiTiarised as follows. 

 IJypnomjjs is represented by two species known by i-eninins from 

 the two largest islands of tlie Balearic gronp and is consideied 

 to belong to the Muscardinidpe, though seeming to be a very 

 distinct form not closely related to any other at present known. 

 Its dental and osteological characters so far observed not only 

 seem to show that Ilypnomys belongs to the Mnscardinidse but 

 also appear to afibi-d further support to the opinion, now very 

 generally held, that this group cannot be included in either the 

 so-called Sciuromorpha or Myomorpha and lending additional 

 weight to the argument against employing these terms. This 

 seems to hold good also for the modern representatives which 

 have been briefly described by Mr. Miller * as "... mostly 

 arboi'eal animals with habits and aspect somewhat intermediate 

 between mice and squirrels. . . ." 



The Balearic genus may be compared with the recent Eliomys 

 and the extinct Leithia from the Pleistocene of Malta. It agi-ees 

 with the former in genei-al plan of skull, lower jaw, and limb- 

 bones, and it may be noted that there is also a perforation in the 

 angle of the lower jaw. It differs in a number of points, among 

 which the following may be cited : — In the pattern of the molar 

 crowns, which are very slightly concave and lack the marginal 

 cusps, both distinctive characteristics of Eliomys. In Hyj^nomys 

 the worn surfaces of the molars are subquadrate in shape, whereas 

 in the recent genus the width is distinctly greater than the 

 antero-posterior length ; the alveolar formula differs in the two 

 genera, and the large inner root in the upper molars of Hypnomys 

 is quite distinctive. This root- structure was evidently attained 

 by a process of simply filling up the space between two parallel 

 roots, whereas the single conical root seen in the Scduridse and 

 others would seem to be arrived at by the drawing together of 

 the apices of the two roots with a similar filling up of the 

 intervening space. It would be interesting could one arrive at 

 a reasonable hypothesis to account for this root-formation in 

 Hypnomys and Leithia, for presumably it must have been acquired 

 to meet a special stress or strain consequent on some peculiaiity 

 of diet or mode of life. 



Ilypnomys agrees with Leithia in its squared molars and in 

 the large, confluent and single inner roots of the upper molars, 

 but differences are seen in the former having a perforation in the 

 angle of the jaw, which is also of a different shape (see text-fig. 1), 

 in the crown pattern of the molars, much lower enamel ridges, 

 and in the roots of the lower molars. 



It has also been noted that in the roots of both upper and 

 lower molars, but not premolars, Uypnomys shows a striking 

 resemblance to Trechomys. However, it is not for a moment 



* ' Cat. Mammals of West. Envope,' London 1912, p. 549. 



