250 THE NATURALIST IN AUSTRALIA. 
of both these forms of oyster growth are characteristically represented in Plate XX XIX. 
of the author’s “ Barrier” book. 
The most notable example hitherto recorded of oysters growing upon trees is 
probably afforded by that delineated in the illustrations on page 249. These portray a 
new and remarkably minute species of oyster discovered by the writer in the estuary of 
the Ord River, Cambridge Gulf, Western Australia, when accompanying, as a guest, the 
surveying cruise of H.M.S. “Myrmidon,” in the year 1888. With reference to its re- 
corded habitat, the name of Ostrwa ordensis was conferred upon this species in a paper 
entitled “ Oysters and Oyster Culture in Australasia,” contributed by the writer to the 
Auckland, New Zealand, meeting, 1891, of the Australian Association for the Advance- 
ment of Science. Ostrea ordensis grows not only on the roots, stems, and respiratory 
shoots, or “cobbler’s pegs,” of the White Mangrove, Avicennia officinalis, but also on its 
leaves. As shown, in fact, in the photographic figures reproducing the specimens their 
exact natural size, as many as forty or fifty individual oysters may be crowded together 
on a single leaf, measuring about two inches in length. That these oysters had attained 
to a state of maturity was established by the fact that, on being opened, they were 
found to be crowded with well-developed embryos. The growth zone of these oysters. 
was, moreover, considerably nearer high-tide mark than that affected by the only other 
type, a stunted race of Ostrwa glomerata, which grew sparingly on the rocks at a lower 
level in the near vicinity. It happens, as a consequence of their inhabiting this high- 
level growth area, that these oysters are covered by the tide for a few hours only 
during the course of the day. During neaps, indeed, they may be left uncovered for 
several consecutive days. There is a famous oyster, celebrated in song, accredited 
with a penchant for walking upstairs. The particular variety here figured has, at any 
rate, manifested the predilection to climb higher up a tree than any oyster species 
previously described. 
A companion picture to the “Rhinoceros Rock” heading to this Chapter has 
been selected for its tail-piece. The rock scene in this instance portrays a somewhat 
remarkable sandstone formation on the foreshore of Sweer’s Island, a member of the 
Wellesley group, in the Gulf of Carpentaria. The anomalous nature of this rock 
formation is recorded in Dr. Fitton’s Geological Appendix to Captain King’s “Survey 
of the Coasts of Australia,” Vol. II., 1826, and is briefly referred to on page 570, 
in the following words :—“<In Sweer’s Island, a hill of about fifty or sixty feet in 
height was covered with a sandy calcareous stone, having the appearance of 
concretions, rising irregularly about a foot above the general surface without any 
