OF EXTINCT MUSCARDINE RODENT. 



217 



specimen of H. mahonensis (B.M. M 11657), in which it is seen 

 to be similar to that of the other two. In some specimens there 

 is a slight groove along the centre, indicating the compound 

 nature of the roots ; the ends are closed in all examples examined. 

 The slight intero-external width is well shown in the section 

 (B.M. M 11661, PI. I. fig. 4) and in the posterior view of the 

 second upper molar (B.M. M 11660, PI. I. fig. 5), in which it can 

 also be seen that there is a certain amount of curvature in the 

 direction of the compound roots. They are considerably longer 

 than the external roots, although, owing to the slope of the 

 crowns, they do not penetrate much deeper into the alveolar 

 ca/ity. 



As already mentioned, the upper molar roots of Leithia are 

 similar to those of Hypnomys, the only other genus, among many 

 examined, showing a closely similar condition of root structure is 

 Trechomys. In the British Museum there are two fragments 

 of the upper jaw of T. platyceps (B.M. M 1627) from the Phos- 

 phorites of Caylux, containing the premolar and first molar, the 

 roots of the latter being as in Hypnomys except that the broad 

 inner root is perhaps not quite so wide compared with the crown. 

 It must be mentioned that in Trechomys the roots of the premolar 

 are the same as in the molars, which is of course not the case 

 in either Leithia or the Balearic genus. The molar roots of 

 Theridomys are somewhat similar but are accompanied by much 

 more hypsodont crowns. 



The roots of the lower cheek-teeth of Hypnomys (B.M. M 11673, 

 PI. I. fig. 7) show a very considerable amount of variation both 

 as regards their length and conformation. The premolar may 

 perhaps be said to have two roots, but these are confluent for 

 the greater part of their considerable length, diverging at a vary- 

 ing but never very great distance from their apices, the anteiior 

 of which is slightly the longer of the two. The upper, posterior 

 portion of the root is w T ide and flattened, being very evidently the 

 result of the fusion of two roots. In some specimens (as in B.M. 

 M 11678) there are clearly three confluent roots. 



In the first and second molars the two anterior roots are 

 usually long, equal in size, and separate though not very widely 

 divergent. In the posterior roots the length and antero-posterior 

 thickness are about the same as in the anterior pair, so that 

 viewed laterally little or no difference is observable (PI. I. fig. 7). 

 But viewed from behind they are seen to be confluent transversely 

 for either the whole or three-fourths of their length (PI. I. 

 figs. 8, 9, B.M. M 11675-6); in the latter case the stout terminal 

 portions of the roots may diverge considerably. The resemblance 

 to Hypnomys seen in the roots of the upper molars of Leithia 

 does not hold good for those of the lower jaw, for in the 

 Maltese genus the first and second molars each have four roots, 

 entirely separate from each other for their entire length. In 

 Trechomys, however, the lower molar roots seem to agree with 

 those of the Balearic genus with the possible exception of the 



