TROCHUS. 17 



others have held. The reduction in number of the teeth in Tro- 

 chidfe appears to i)roceed by the h)ss of the outer laterals, never by 

 obsolescence of teeth in the central portion of the radula. (Compare 

 the related ftimily Turbinidse, where the rule is specialization of the 

 radula by the loss of the central tooth, as in Orthoviesus, or by the 

 loss of its cusj), as in Bolma, Cookia and Pomaulax; in the last group 

 the inner lateral also has lost its cusp. Vide Manual, vol. x, pp. 

 163, 187.) PI. 50, fig. 1, Trochus niloticus L. ; pi. 50, fig. 2, T. 

 fenestratus Gmel. ; pi. 50, fig. 3, T. maculatus, L. ; pi. 50, fig. 4, T. 

 tiaratus Q. and G. 



The central teeth in all of the subgenera of Trochus, including 

 Clanculus, differs from those of the following genus, Mondonta, in 

 being widest in the middle, tapering toward the cusp and the base ; 

 whilst in jMonodonta and its subgenera the latero-basal angles are 

 prominently ])roduced. 



The operculum (pi. 1, £ 6) is circular, corneous, thin, with num- 

 erous (8 to 12) narrow whorls, the nucleus central. 



Subgenus Trochus (s. str.) 



T. NILOTICUS Liune. PI. 1, figs 5-8. 



Shell large, ponderous, conit*al, appearing subperforate, covered 

 by a corneous striate, brown or yellowish cuticle usually lost on the 

 upper whorls ; colorbeneath the cuticle white, longitudinally strij^ed 

 with crimson, violet or reddish brown, the base maculate or radiately 

 strigate with a lighter shade of the same ; spire strictly conical, apex 

 acute, usually eroded, whorls 8-10, the upper ones tuberculate at the 

 sutures, and spirally beaded, the following flat on their outer sur- 

 faces, smooth, separated by linear sutures, the body-whorl expanded, 

 dilated and compressed at the obtuse periphery, more or less convex 

 below, indented at the axis; umbilical tract covered by a spiral 

 pearly deeply entering callus; aperture transverse, very oblique; 

 columella oblicjue, terminating in a denticle below, and with a strong 

 spiral fold above, deeply inserted into the axis. 



Alt. 80-100, diam. 100-120 mill. 



Indian Ocean; New Ireland; Neio Caledonia; North Australia, 

 etc. 



Trochus niloticus L., Syst. nat. xii, 1767, p. 1227. 



Operculum pi. 1 fig. 6, circular, thin, corneous, orange-brown, coin- 

 posed of about 10 whorls. 

 2 



