COMPOSITION AND ORIGIN. 205 
evidence offered by brittle-stars in support of the eastern origin of the Barrier 
Reef group. While 15 species and one well-marked variety of ophiocomids occur at 
Mer, and 12 of these are known from farther east (3 are at present apparently 
endemic), only a single species (Ophiarthrum elegans) is known from the Thursday 
Island area, only one (Ophiocoma pica) from the Kei Islands, and only 3 (Ophiocoma 
brevipes, scolopendrina, and wendtii) from northwestern Australia. In view of the 
abundance of these brittle-stars at the Murray Islands, their large size, conspicuous 
appearance, and active habits, it is incredible that their apparent absence from the 
Thursday Island region is due to superficial collecting there. Five forms were 
found at Green Island, and at least one other is known from the Queensland coast, 
but we are still very much in the dark as to how far southward along the Barrier 
Reef these fine ophiurans have extended their range. The occurrence of Ophiar- 
thrum elegans in the Thursday Island region can easily be accounted for as a local 
westward extension of range from the Barrier Reef, and its absence from the Ara- 
fura Sea and the adjoining regions northward and westward confirms this view. 
The occurrence of Ophiocoma pica at the Kei Islands and along the coast of Timor, 
considered in the light of its absence from the Aru Islands and Arafura Sea, is 
unquestionably due to a southern movement from the East Indies. In the case of 
O. scolopendrina and wendtii, such a movement has continued across the Sahul 
Bank to northwestern Australia. 
Most of the 11 echini included in the Barrier Reef group occur to the west of 
Torres Strait, and hence do not offer very satisfactory evidence concerning their 
origin. Of the others, Echinostrephus is so secretive a form that its apparent 
absence from any area may easily be due only to its having been overlooked. Stomo- 
pneustes is also secretive, but its large size makes it much less likely to be overlooked, 
and its entire absence from the region west of the Murray Islands, taken in connec- 
tion with its occurrence at Samoa and on the Queensland coast, indicates its Pacifie 
origin. The three spatangoids, Metalia, Brissus, and Maretia, have a distribution 
like that of Stomopneustes, although Brissus alone was met with at Mer. Both 
Brissus and Maretia extend their range as far as Port Jackson, which would suggest 
that they were among the first echinoderms to reach the Australian coast after 
the connection with the Pacific was established. 
There are 15 holothurians in the Barrier Reef group, and each (excepting half 
a dozen species of Holothuria) represents a different genus, but 8 species occur 
west of Torres Strait, and hence their place of origin is not indisputable. The other 
7 occur in the East Indies, but 6 of them are common in the Pacific. A typical 
example of the group is Polyplectana kefersteinii, which occurs at Hawaii as well 
as at many of the more western islands. Apparently none of the holothurians 
from the Pacific influx has yet reached Port Jackson, and only 5 are listed from the 
Queensland coast. But Leptosynapta dolabrifera, the endemic species of south- 
eastern Australia, is undoubtedly the descendant of a leptosynaptid which was 
an early arrival from the Pacific. 
We find, then, that in what is here called the Barrier Reef group of echino- 
derms, there are at least 40 species, more than half the group, whose distribution 
