THE ECONOMICS OF TKOCHUS N1LOTICUS.
By CHARLES HEDLEY.
The following was written as a Report from the Special Committee on Marine Biological Economics of
Tropical Australia, appointed by the Commonwealth Institute of Science and Industry.
This large and handsome shell was mistaken for a product of the River Nile by Aldrovandus, who.
in 1606, was the first writer in Europe to describe it. Thus Linnaeus in 1767 adopted from him the
title of Trochus niloticus. Other scientific names that it has since received are Trochus spinosus Gmelin,
1791 ; Trochus flammeus Bolten, 1798; Trochus zebra Perry, 181 1 (Mathews & Iredale, Victorian
Naturalist, xxix., 1912, p. 13) ; Trochus marmoratus Lamarck, 1S22 ; Astralium pagodus Wood, 1879
(Hedley, Proc. Linn. Soc. N.S. Wales, xxxiii., 190S, p. 467) ; and Trochus montebelloensis Preston, 1914.
In the Philippine Islands it is popularly known as " Chin leh," and at Cape Bedford, Queensland, the
aboriginals call it " Dobbi."
Trochus niloticus had long been considered (Lamarck, Syst. An. s. vert., 1801, p. 85) as the type of
the genus Trochus. But Iredale (Proc. Malac. Soc. x., 1912, p. 225) notes that not being one of the
original party, it is inadmissable, and designates T. maculatus as the type. As a sectional name
Pyramidea, Swainson, may be appropriated by T. niloticus.
Description of the Shell.
The remarkable feature of T. niloticus is the grotesque expansion of the last whorl. In the
related T. maximus, not yet recorded from the South-west Pacific, the normal angle of the spire is
continued as usual to the last. But in T. niloticus a bulge commences in the penultimate whorl, and
increases rapidly, carrying the last whorl out of alignment with the rest. So that the last whorl
approaches the horizontal, and the aperture, from being twice as broad as high, becomes three times
as broad as high. Finally the insertion of the lip tends to drop below the periphery.
As with other large species, the summit is so severely eroded that the upper whorls cannot be
counted on any adult individual. By combining measurements of a young, of a half grown, and of
a large shell, I arrived, as follows, at an estimate of fourteen whorls for a complete specimen. In the
youngest example used, having a maximum diameter of fifteen millimetres, the earliest, or at least one
whorl, had already vanished. The pr.sumed second whorl is 1 millimetre in diameter, the
third ij; the fourth 2; the fifth 4; the sixth 5 ; the seventh 7 1 ; the eighth 12 mm. Now
changing to the medium shell, that whorl which has a diameter of 12 mm. is presumed to be
the eighth; accordingly in this individual the seventh whorl is 8 mm.; the eighth 12; the
ninth 19 ; and the tenth 30. Again changing to the largest shell, that whorl which has a
diameter of 30 is regarded as the tenth, and thus proceeding, the ninth whorl has here a dinmeter of 20 ;
the tenth 30 ; the eleventh 42 ; the twelfth 64 ; the thirteenth 91 ; and the last and fourteenth 142
mm. (say $i inches). The minor diameter of this measured shell (Figs. 1, 2) from Samarai, Papua,
is 123 mm., the height 120 mm. ; the weight is a pound and a half. No such size has, so far as I am
aware, been recorded in literature. Fischer (Monogr. Tiochus, 1880, p. 67), gives the breadth as 140
mm., the height as 95 mm. The largest specimen which von Martens had examined was only 124 mm.
in breadth ; this he contrasted with a dwarf only 61 mm. high and 67 broad (Martens, Ann. Mag. Nat.
Hist. (3) xx. 1867, p. 99). Other Samarai shells have, a diameter of 141, 137, 135, and 129 mm., and
another from Torres Strait is 133 mm. The correspondent, to whose kindness I owe this material,
writes of giants from Samarai of eight inches diameter, which he has seen. Such would probably have
an additional half whorl.