1906.] MAMMALS FROM WESTERX AUSTRALIA. 7^3 



mammals in this part of tlie countiy is very curious, as, with all 

 these Rat- Kangaroos on the islands ofi' the coast, they are entirely 

 absent from the mainland about here (Caiiiarvon). [It remains 

 to be seen, however, whether there are none on the coast to the 

 south, which a study of the map would indicate as the natural way 

 of entry for the animals of these islands. — 0. T?^ In addition to 

 the species sent, the island possesses Perameles hougainvillei, of 

 which there is an example from ' Denham Sound ' in the Perth 

 Museum : but I was unable to secure a specimen, though I picked 

 up a dried skull, and I fear that, owing to the presence of cats, 

 they may have been exterminated." 



The specimens sent by Mr. Shortridge from Berniei' Island prove 

 to be most interesting ; for in eveiy case they are definably different 

 fnmi the S.W.-Australian form to which he supposed them to 

 belong, and from which they would appear to be widely separated 

 geographically. 



And in this differentiation there is one interesting and noticeable 

 point, namely, that all three of the Rat- Kangaroos diflfer from 

 their respective allies in one character — the compai-ative shortness 

 of their ears and a correlated reduction of their auditory bulla^. 

 When we remember that the forms affected belong to three quite 

 distinct genera, this instance of geographical isomorphism is well 

 worthy of mention. 



As a cause it may be suggested that since, as Mr. Shortridge 

 states, the animals have on the island no enemies to fear, the 

 faculty of hearing would have lost that supreme imj)ortance foi- 

 the preservation of life that it would have had in the joresence of 

 man, dingoes and dasyures. The ears would have consequently 

 tended to become reduced by the survival of individuals with 

 duller hearing, who in other places would have been speedily 

 eliminated by predatory enemies. 



43. Bettongia lesueuri Quoy & Gaim. 

 Three males ; six females. 



These specimens I'epresent the typical lesueuri, which was dis- 

 covered during the Voyage of the ' Uranie ' on the neighbouring- 

 island of Dirk Hartog, and, as in the case of Lagostrophus fasciatus, 

 prove, on comj^arison with the good series obtained by Mr. Short- 

 ridge in Avon County, to be recognisably different. Their ground- 

 colour is paler, their fur is less long, and their ears (just as in the 

 case of the Lagostrophus) are very distinctly shorter. The fol- 

 lowing are Mr. Shortridge's measurements of a pair from each 

 region : — 



B. lesv,euri lesueuri. — Bernier Island. 



Head and body. Tail. Hind foot. Ear. 



mm. mm. mm. mm. 



6 350 280 102 35 



$ 360 300 110 36 



B. lesueuri grayi. — Avon District. 



S 390 310 112 42 



$ 360 285 108 40 



