70 THE ECONOMICS of Trochus niloticus. 



Nothing definite is known of the age of T. niloticus. According to native tradition in Fiji, the 

 mollusc lives for four years ; the youngest stages are passed in deep water on the reefs, a shell at two 

 inches diameter is thought to be six months old, at three inches one year, and at four inches two years, 

 the rate of increase becoming slower as growth proceeds. 



The upper and lower whorls are diversely sculptured. For the first eight whorls, in the pagodus 

 stage (Fig. 5) the periphery carries projecting, imbricating, hollow, arched scales at about 14 to a 

 whorl. A transition stage, that of spinosus (fig. 4) follows, in which, for a whorl or two these scales 

 are gradually modified into obliquely compressed tubercles and then vanish, leaving the peripheral keel 

 quite smooth. In this family, such scales are a usual feature, reaching espechl development in A strea 

 heliotr opium, Guildfordia triuinp/ians. Angaria delphinula and Turbo marmoratus. Even when lost 

 in maturity, these tend, as here, to appear in infancy. They also do so in the case of Turbo stamineus 

 (Kesteven, Proc. Linn. Soc. N.SAV., xxvi., 1902, p. 175, PI. xxxv.. f. 2). It is now suggested that 

 these peripheral scales may be a reminiscence of the pore-scales of Haliotis. In respect of these scales 

 T. niloticus is further developed than such co-generic types as T. nodiferus and T. dev.tatus where the 

 scales persist throughout life. On the upper whorls the area between the peripheral scales and the 

 channel'ed suture is occupied by four or five spaced lyrae, the two upper nodulous, the others flat. On 

 succeeding whorls the lyrae first multiply and then disappear gradually. At eight whorls the baes 

 has about fourteen fine, spiral, lyrae, increasing by intercalation and as broad as their interstices. 

 These continue of the same relative importance to the eleventh whorl, after which they gradually 

 fade till scarcely a trace can be distinguished on adult shells. 



The axis is a cork-screw shaped columella, the last turn of which is free in the axial funnel, the 

 remainder embedded in the roof of callus above. Below this plait the columella is produced in a longer 

 spiral to a broad, horizontal, knife-like process at the base of the aperture. In young shells the false 

 umbilicus is a narrow, deep, spiral funnel with steep sides, but in senile examples the hollow is filled 

 with a callus pad, nearly flush with the rest of the base. The centre and columella are nacreous, a 

 crescent opposite the aperture is porcellaneous. A similar succession is seen in the inner lip, where 

 the border is porcellaneous and the interior is nacreous. 'When the shell has been boiled, the fact is 

 shown by superficial, microscopic cracks in the nacre of the axis. The right and left insertions of the 

 lip leave between them an uncovered space of the previous whorl. Only the last four whorls of the 

 adult shell are inhabited, the earlier ones being filled solid with porcellaneous callus. (Fig. 3). 



The colour is a white ground painted with zic-zac red flames as broad or broader than their inter- 

 stices and descending forwards across the line of growth, and ranging from light pink to dark purple. 

 On the base these flames sometimes break up into cuneiform marks with the points directed backwards. 

 Within the aperture red is often replaced by olive-green, but this is not, as von Martens supposed, a 

 result of the original colour fading. On the upper whorls are a subsutural row of six or seven purple 

 patches, between which are close, narrow, rose-pink lines' sometimes appearing on the lyrae as articula- 

 tions or arrow heads. 



In life, the whole shell, except the axial area, is clad in a fibrous epidermis, brittle when dry, frilled 

 into small lamellae set at four to the millimetre. These run down backwards, along the lines of growth 

 but across those of colour. This epidermis does not intrude into the axial funnel. 



The operculum (fig. 7) is a thin, flat, round, chitinous plate, 32 mm. in diameter, composed of 

 numerous narrow spirals. Below, it is glazed with chitin deposited after the growth of the spirals 

 and showing a line of muscular attachment. In the centre is a minute pivot knob, on which the whole 

 operculum revolves. On the upper surface is a small, corresponding axial pit. These features- 

 seem not to have been previously observed. 



The upper side is in life probably raised in a low cone, for the middle is abraded. For this reason 

 it is impossible to count the whorls which obviously exceed ten. Probably the operculum corresponds 

 whorl for whorl with the shell, thus making fourteen. On the outer spiral is a dark border and a fringe 

 along which a subsequent spiral would be wound. 



