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Show 862 ON THE EXTINCT GIANT DORMOUSE OF MALTA. [Dec. 3, maxilla, situate considerably in advance of the anterior zygomatic root. N o w in this respect the fossil apparently differs from the Dormice and resembles the Squirrelsl. As regards the cheek-teeth, these present a considerable superficial resemblance to those of such Dormice as have complicated enamel-folds on the crown. Closer examination shows, however, a marked difference, well displayed in the accompanying figures (p. 861). In the simpler type of upper molars in the Dormice (Eliomys) the folds form ridges, of which two unite to form columns on the outer side of the crown, so as to give a somewhat tritubercular form to the whole tooth; and where the ridges are more complex (Myoxus), and form more distinct plates, these curve together in pairs on the outer side of the crown. On the other hand, in the Maltese animal the four or five blunt ridges diverge from the inner or postero-internal portion of the crown in a comb-like manner, without any tendency to approximate on the outer border, one or two of these ridges being shorter than the rest and arising by a splitting of the enamel-folds. Both Eliomys and Myoxus have distinct outer columns to the upper molars, while Muscardinus differs markedly from the fossil in that the upper premolar is very small and the first molar much longer than the second. The foldings, too, on the second molar are much finer and of a different type- Comparing this type of dentition with that of the Sciuridee, the nearest approximation is presented by Xerus and Pteromys. In the former two main ridges and two talons proceed outwards from an inner wall; and in the latter this inner wall becomes thinner, aud the two talons are so developed as to be counted as ridges. Although these ridges and the three intervening enamel-folds, or valleys, are much deeper than in the Malta form, yet it seems not impossible that both types of teeth might be derived from a common stock. Seeing, therefore, that the Maltese rodent does not belong to the Myoxidee, while it presents certain resemblances to the Sciuridee, I think it may be provisionally assigned to the Sciuromorpha, although it is quite probable it may constitute a family (Leithiidce) by itself. As it requires a new generic title, the name Leithia, after its describer, m ay be suggested; and the genus may be provisionally defined as specialized Sciuromorpha with squared upper molars bearing from four to five simple, low, parallel transverse ridges, three of which rise from the postero-internal angle or inner side of the crown, while the remainder are shorter and are formed by a single or double splitting of the more anterior of the two hind- 1 As pointed out to m e by Mr. de Winton, the palatine foramina differ from those of existing Sciuromorpha in extending backwards well into the maxillae, as in the Myomorpha, instead of being confined to the premaxilla?. In Arctomys, however, these foramina intrude somewhat into the maxillre, so that the feature does not seem of much value. Neither a m I inclined to attribute very much importance to the distal union of the tibia and fibula which takes place in the fossil form, although such union is unknown iu the living Sciuromorpha, while it is universal in the Myomorpha. |