36 Christmas Island. 



its homologue in Pedetes is a dermal bone of secondary origin. 

 In the latter gemis the radial pad of other Eodents has been 

 transformed into the volar prominence sheathed with a homy cap ; 

 in other words, the structure which in most Rodents is an organ of 

 touch has been adapted to fossorial functions in Pedetes} 



It is not clear to me on what grounds Emery here denies 

 a cartilaginous condition to the distal element of Mus decumanus. 

 In a subsequent memoir- he seems disposed to adopt a different 

 view. After having shown that in Marsupialia the ' prsepollex ' 

 is ' tj'pically ' composed of two elements, he adds : "It will be of 

 particular interest to investigate whether in other mammals, 

 provided with a compound prsepollex, the distal element has 

 a cartilaginous Anlage (' knorpelig angelegt wird '), and is the 

 homologue of the distal portion of the marsupial prsepollex." 



Like all Muridse which have come under my observation, 

 both Mus nativitatis and Mus maeleari have a large- sized marginal 

 radiale, articulating Avith the ' scapholunar ' and the Metac. i on 

 their median and volar side, and thence extending obliquely across 

 the vola in the direction of the distal pisifonn bone. In both the 

 species there is attached to the distal and ulnar margin of the 

 marginal radiale, what appears to be a laminar cartilaginous 

 appendix, incompletely ossified on the ulnar side. Neither of the 

 two specimens — one from each species — is adult. I expect that in 

 adult specimens the ossification will be found to extend over the 

 whole of the distal part also. This condition I find to be the case 

 in an individual of a genus closely allied with Mus^ viz. in 

 Arvicanthis {A. niloticus), although the sj)ecimen is not pei'fectly 

 adult. 



The principal differences in the skulls of the two specimens have 

 been pointed out by 0. Thomas (P.Z.S., 1888, pp. 533, 534), and 

 are borne out by the figures of Plate II {lis). The skull of 

 M. nativitatis is weaker, more slender, and narrower than that 

 of the other species ; the outer wall of the infraorbital foramen 

 is shorter and projects less forward ; the incisive foramina extend 

 farther backwards. The molars of this species likewise are weaker 

 and less complicated with secondary cusps. In M. maeleari, the 

 outer cusps of the upper and the inner cusps of the lower molars 

 are much stronger than the corresponding parts in M. nativitatis. 

 In several specimens of the latter the teeth present a diseased 

 appearance, which is not the case in any of the specimens of 

 M. maeleari. I append the measiirements in millimetres of some 

 of the skulls collected by Mr. Andrews, three of M. maeleari and 

 two of M, nativitatis. 



^ C. Emery, "Zur Morphologie des Hand- und Fuss-skelets " : Anat. Anz., 

 V, pp. 28S-291 (1890). 



- C. Emery, "Beitrajje z. Entwickhmgsgesch. und Morphologie des Hand- und 

 Fuss-skelets der Marsupialier " : Semen's Forschungsreisen, ii, p. 394 (1897). 



