216 



MISS DOROTHEA BATE ON A NEW GENUS 



Hypnomys and Leithia, but an examination of the roots them- 

 selves show them to be essentially different. 



In Hypnomys the upper premolar has three long and rather 

 stout roots, which terminate in blunt rounded ends (B.M. M 11 658, 

 PI. I. fig. 3). The anterior one takes a forward direction and is 

 the shortest of the three, the other two project in a parallel 

 direction from the hinder border of the tooth and may be con- 

 fluent for a varying distance, but apparently not for more than 

 half their length. This description is based on examples of 

 Hypnomys mahonensis, but the alveolus in a Mallorcan specimen 

 indicates a similar formation. Unfortunately there has been no 

 opportunity of examining any milk-teeth in the case of either 

 this genus or Leithia. 



Text-figure 2. 



A. B. 



m 



1 v § si V 



[/.alveoli. LaJveoli. U. alveoli. LMveoli 



Diagram of alveoli of cheek-teeth of A. Hypnomys and B. Leithia. 



The upper molars each have three roots, one large internal and 

 two small external ones, a number commonly found in many 

 rodents. The distinctive feature of the Balearic teeth lies in the 

 construction of the large internal roots: in Eliomys, Glis, and 

 also in other forms such as Xerus and Sciurus, which have the 

 same alveolar formula, these are single, rather flattened conical 

 roots projecting from the centre of the border of the crown which 

 they support. In Hypnomys the internal root is quite different, 

 this being clearly portrayed in the accompanying illustrations. 

 Fig. 3 (PL I.) shows the latero-internal view of the first and 

 second molars of H. mahonensis : it will be seen that there is a 

 slight constriction at the base of the crown where the enamel 

 ends, and from there the confluent root continues practically of 

 the same size for its whole length, which is a little less than twice 

 the antero-posterior length of the crown-surface. In the figured 

 specimen the first molar measures 4 mm. from the crown-surface 

 to the tip of the root, while the antero-posterior length of the 

 crown is very little more than 2 mm. In the second molar the 

 root is very slightly shorter and not rounded at its apex, but this 

 may perhaps be partly due to damage in developing the specimen. 

 The internal root of the third molar is exposed in the type 



