Yol. 50.] FATOA FROM THE IGHTHAM FISSURE. 195 



the common living species. The largest of these rami measures 

 about 15*5 millim. from the point of the incisor tooth to the arti- 

 cular condyle ; while the three cheek-teeth occupy about 3*6 millim. 

 and the front tooth 1*7 millim. — measurements which agree with 

 those of M. sylvaticus. One or two front upper cheek-teeth are 

 preserved, which agree with this species. Mus sylvaticus has been 

 found in the Forest Bed, is now living in nearly the whole of 

 temperate Europe as well as in Britain, and is found in Western 

 Siberia and the Caucasus. In the Alps it extends upward to 6000 

 feet above the sea. 



Mus Abbotti, sp. nov. PL XI. fig. 8. — Seven "mandibular rami 

 and parts of four skulls, considerably larger than those referred to 

 M. sylvaticus, it was at first thought might belong to one of the small 

 species of Cricetus l ; but fortunately one of the rami has all the teeth 

 preserved, and a second has the front cheek-tooth in place ; these 

 teeth are found to agree with Mus, and not with Cricetus. All the 

 cheek-teeth are well worn, and the pairs of cusps have united late- 

 rally, but the longitudinal groove is deep and similar to that in 

 M. sylvaticus. The front cheek-tooth wants the anterior accessory 

 cusp, or has it very small ; in one example it has united with the 

 anterior pair of cusps, while in another specimen it is so small as 

 only to be seen with difficulty. The hinder and outer accessory cusp 

 is as in M. sylvaticus, though the accessory cusp opposite the median 

 pair is reduced to a compressed and elongated ridge, as in some of 

 the specimens referred to M. sylvaticus, and like that which obtains 

 in M. minutus. In M. sylvaticus this same cusp sometimes has a com- 

 pressed appearance and is little more than a part of the outer cingulum. 

 The second and third cheek-teeth in the present form are like those 

 of M. sylvaticus. Pour portions of skulls are provisionally associated 

 with these lower jaws ; they are without teeth, and are not only 

 larger than in M. sylvaticus, but have proportionately wider palates, 

 shorter noses, and broader interorbital spaces. The teeth of this 

 mouse agree most nearly with those above referred to Mus sylva- 

 ticus, differing, however, from that species in the absence or slight 

 development of the anterior accessory cusp of the front lower 

 cheek-tooth. The length of the best-preserved ramus, from the 

 point of the incisor to the articular condyle, is 18*8 millim. ; the 

 entire series of cheek-teeth measures 4*1 millim., and the anterior 

 cheek-tooth 1*8 millim. The only fossil species that need be men- 

 tioned is the Mus orthoclon, Hensel, 2 the teeth of which, so far as 

 one may judge from the much-worn examples which are figured, 

 approximate to the present fossil ; but as the front cheek-tooth 

 of that species is one third larger than that of the specimens 

 now being considered, the two cannot well be referred to the same 

 species. I propose to name this new form Mus Abbotti. 



1 See Nehring, 'TJeber Pleistocane Hamster-Keste,' Jahrb. d. k.-k. geol. 

 Reichsanstalt, vol. xliii. (1893) p. 179. 



2 Zeitsehr. Deutsch. geol. Gesellsch. vol. viii. (1856) p. 281. See also Dr. 

 Forsyth Major, Atti Soc. Toscana di Sci. Nat. Proc. Verb. vol. iv. (1881) p. 129. 



