THE ECONOMICS OF TKOCHUS N1LOTICUS. 

 By CHARLES HEDLEY. 



(Plates v.-vi.) 



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. 



Nomenclature. 



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. 



