1888.] OF THE SOLOMON ISLANDS. 481 



each other specifically, they have a great many characters in common, 

 as for example their considerable size, the general forms of their 

 skulls and teeth, and, notably, their very unusual mammary formula, 

 — 2 = 4, a formula only found, so far as I know, in two other 

 members of the genus, namely Mus trivirgatus, Temm., and M. 

 delicatulus, Gould, both quite small species, and the latter nearly, if 

 not quite, the most diminutive member of the genus. 



It is, however, in their relation to each other that their chief 

 interest lies, for they seem to be clearly the slightly modified descen- 

 dants of one single species that, once introduced, has been isolated in 

 Guadalcanar for some considerable time, while it has apparently died 

 out elsewhere. Of this original species some individuals would 

 have adopted a terrestrial, and others an arboreal life, and their 

 respective descendants would have been modified accordingly. In 

 this way I would explain the fact that at the present time we have 

 in Guadalcanar two genuine species, agreeing with each other in 

 their essential structure, and yet separated by a considerable number 

 of characters all having a more or less direct relation to a climbing 

 or non-climbing habit of life. Of these, of course by far the most 

 striking are the broad foot-pads and the long, rasp-like, probably 

 semi-prehensile tail of Mus rex, as compared to the smaller pads 

 and short smooth tail of Mus imperator. 



20. Mus sALAMONis, Rams. 



Mus salamonis, Rams. P. Linn. Soc. N. S. W. vii. p. 43 (1882). 



I know nothing more of this species than is contained in the 

 original description. Its hind foot is stated to be 44 millim. in 

 length, and its locality is Florida Island. The length of its hind 

 foot, curiously enough, exactly fills in the gap between that of M. rex 

 and of the next species, so that the four Solomon Island Rats have the 

 lengths of their hind feet just in the progressive series 34, 44, 54, 

 and 64 millim. 



21. Mus PR^TOR, Thos. (Plate XXII. fig. 6.) 

 Mus prtetor, Thos. t. c. p. 158. 



a, b. S 2 ' Aola. 



Size about that of Mus rattus. Fur short, mixed with numerous 

 spines, and with a few much longer piles on the posterior back. 

 General colour coarsely grizzled grey, the longer piles and the spines 

 black-tipped, the ordinary fur with yellow tips ; the bases of all 

 pale slaty grey. Underside dirty white, in old specimens yellow ; 

 the hairs all grey at base. Ears rounded, rather short, laid forward 

 they just reach to the posterior canthus of the eye. Mammae 

 2 — 2=8. Hands and feet greyish white, a darker patch on the 

 terminal part of the metatarsus. Hind feet (Plate XXII. fig. G) 

 rather short in proportion to the size of the animal ; fifth toe reach- 

 ing to the middle of the first phalanx of the fourth ; soles naked, 

 the pads rounded, rather small. Tail short, not so long as the body 

 without the head, thinly haired and coarsely scaled, the rings of scales 

 averaging about 10 to the centimetre ; its colour wholly deep black. 



