SYSTEMATIC REVISION OF THE AUSTRALIAN THORNBILLS 87 
Of the three species which occur in Tasmania, only A. ewingi 
is absent from the mainland, and it is worthy of note that it is 
the only species of the genus represented in recent collections 
from King and Flinders Islands, Bass Strait. 
To provide a basis for comparison between races it has 
been necessary to describe in full a typical example of each 
species; accordingly the typical form in each instance is 
included first irrespective of its geographical position. 
Colour terms are from Ridgway’s Color Standards and 
Nomenclature, and all measurements are in millimetres. 
Genus ACANTHIZA Vigors and Horsfield. 
Acanthiza Vigors and Horsfield, Trans. Linn. Soc., xv, 1927, p. 244. 
Type by original designation, Motacilla pusilla White. 
Geobasileus Cabanis, Mus. Hein., i, 1851, p. 32. Type, by monotypy, 
Saxicola chrysorrhoa Quoy and Gaimard. 
Milligania Mathews, Aust. Av. Rec., i, 1912, p. 112. Type, by original 
designation, Acanthiza robustirostris Milligan. 
Subacanthiza Mathews, Birds of Aust., ix, 1922, p. 449. Type, by 
monotypy, Acanthiza lincata Gould. 
The original description of Geobasileus is not available to 
me, but authors who have accepted that genus have stressed 
the difference in size and wing formula, and the bright colour 
of the rump compared with Acanthiza. The difference in size 
is slight, chrysorrhoa having a longer wing and bill, but 
shorter tail and tarsus compared with the type of Acanthiza, 
and the only difference in wing formula is that in chrysorrhoa 
the third primary approaches nearer in length to the fourth 
than in pusilla. The main distinguishing feature of Geo- 
basileus is the distinct colour of the rump, but this does not 
warrant generic rank. Indeed, a striking difference in colour 
of rump is characteristic of other species of the group, and is 
a trenchant subspecific character between races of pusilla. 
The retention of Geobasileus on this single feature has led to 
peculiar divisions of the species by some authors. Because of 
the light buff colour of the rump it has been customary to refer 
reguloides to Geobasileus, and for many years Mathews 
included hedleyi (= A. iredalei hedleyi ) as a species of that 
genus, while retaining iredalei in Acanthiza. 
The two species for which Milligania and Subacanthiza 
were erected in no way differ from Acanthiza, and the author 
of both names recently retained only Acanthiza where for- 
merly he accepted three genera and one subgenus (Syst. Av. 
Aust., p. 601). 
