Mammalia. 35 



The above remark does not, however, ap])ly to the ungual 

 phalang-es, vrhieli in M. naticitatis are ioniJi-er, stronger, and less 

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

 more striking, as -with regard to the proxinial phalanges and 

 nietapodials the proportions are reversed, these being more elongate 

 in J/, madeari, as already pointed out. 



The ungual phalange of the first digit of J/", ^lativitatis is 

 not diffei'ent in character from those of the other digits. In 

 M. madeari the whole of the thumb, including the ]Metac. i, is 

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

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

 like, curved ungual i)halanges in the other fingers, and resembling 

 somewhat the same element in Primates. A still more appropriate 

 comparison of the digits of the arboreal M. madeari is with that 

 of Sciurus (e.g. S. vulgaris), 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, howe'S'cr, in the squirrel, ])roceeded further than in 

 31. madeari; ^\'hilst the third and fourth digits have further 

 ])roceeded in the op])osite sense, being disproportionately long. 



In both M. nativitatis and 21. madeari, an ossicle overlies, 

 dorsally, the interphalangcal 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 Rodentia and Insectivora, may be the second 

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



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

 the distal pisiform of Murida?, etc. ; this bone forms, so to sa}-, 

 a 'pendant' to the distal ' pra^pollex ' (see below); it occurs in 

 both the species. See Text-tig. 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. 



Marginal radiale. — The only writer, to my knowledge, who has 

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

 who describes it in 2f. decumanus. In this species there is in 

 connection with the distal extremity of the bone in question 

 a lamina of tendinous connective tissue having cartilaginous con- 

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

 and acting as a siipport to the; very prominent and compact radial 

 pad. The single bone of Mas decumanus 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 Mus decumanus. The character of a cartilage being 

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



