Mammalia. 35 



The above remark does not, however, apply to the ungual 

 phalanges, which in M. natkitatis are longer, stronger, and less 

 curved than in the other species ; the difference in length is the 

 more striking, as with regard to the proximal phalanges and 

 metapodials the proportions are reversed, these being more elongate 

 in 21. macleari, as already pointed out. 



The ungual phalange of the first digit of JI. nativitatis is 

 not different in character from those of the other digits. In 

 M. macleari the whole of the thumb, including the Metac. i, is 

 more reduced than in the former species ; and its ungual phalange 

 has a rounded, nail-like form, absolutely different from the claw- 

 like, curved ungual phalanges in the other fingers, and resembling 

 somewhat the same element in Primates. A still more appropriate 

 comparison of the digits of the arboreal 31. macleari is with that 

 of Sciurm (e.g. S. vuiffaris), where we find the same curved, claw- 

 like, ungual phalanges of digits ii-v, and the same rounded, 

 nail-like, ungual phalange of the first digit. The reduction of the 

 thumb has, however, in the squirrel, proceeded further than in 

 M. macleari; whilst the third and fourth digits have furthei- 

 proceeded in the opposite sense, being disproportionately long. 



In both M. nativitatis and 21. macleari^ an ossicle overlies, 

 dorsally, the interphalangeal articulation of the thumb. I have, 

 on a recent occasion, P.Z.S. London, 1899, p. 430. suggested that 

 this ossicle, which, although never mentioned before, is of quite 

 common occurrence in llodentia and Insectivora, may be the second 

 phalange of the thumb, having been thmst out on the dorsal surface. 



In the same place, quoted before, I have treated at length of 

 the distal pisiform of Muridte, etc. ; this bone forms, so to say, 

 a ' pendant ' to the distal ' proepollex ' (see below) ; it occurs in 

 both the species. See Text-fig. 5, op. cit. 



The ossicle, which in the figures given in the above quoted 

 paper is marked x, is equally present in both species from 

 Christmas Island, situated on the volar side, between the latero- 

 distal angle of the radius and the pisiform; in old specimens 

 it becomes fused with the former, but it is quite possible that it 

 often vanishes. 



In M. nativitatis it is much smaller than in the other species. 



31argi)ial radiale. — The only writer, to my knowledge, who has 

 made mention of the ' praepollex ' in the genus Mus, is Emery, 

 who describes it in J/, decumaniis. In this species there is in 

 connection with the distal extremity of the bone in ([uestion 

 a lamina of tendinous connective tissue having cartilaginous con- 

 sistence ("eine knorpelharte Platte von sehnigem Bindegewebe "), 

 and acting as a support to the very prominent and compact radial 

 pad. The single bone of Mas decuinanus is considered to be the 

 homologue of the proximal of the two bones occurring in Pedetes ; 

 to the distal bone of the latter would correspond the ' tendinous 

 lamina' of Mas decumanus. The character of a cartilage being 

 denied to the latter structure, the inference drawn from this is that 



