92 DISCOVERY REPORTS 



Genus Venustatrochus n.g. 

 Type : V . georgianus n.sp. 



This genus is provided for a benthic species from South Georgia, which on external shell features is 

 not dissimilar from normal deep-water thin-shelled Calliostomids. The dentition, however, is so 

 different from that of typical Calliostoma, Phot inula and the Photinula-Yike Calliostomids of the Magellan 

 Province that generic separation is essential. Normal Calliostomids exhibit a radula with from four to 

 seven laterals and a central tooth with a simple, more or less rectangular base, which is deeper than it is 

 broad. The inner marginal is massive, long and somewhat crooked at the end, and bears on the under- 

 side a small number of prominent comb-like denticles. The central and laterals have long and slender 

 curved cusps which are delicately serrated, and it is difficult to imagine how they can be effectively used 

 without suffering damage. 



The radula of Venustatrochus resembles that of the Calliostomids in the possession of large inner 

 marginals with comb-like cusps and elaborately serrated, slender, curved cusps on the central and 

 laterals. The discordant features are shown in the central tooth, which has a long, very narrow cusp on 

 a large diamond-shaped base, and in the laterals, which are as the central except for being cut away on 

 the inner side of the base. The most remarkable difference is in the number of laterals (sixteen) which 

 is greater than in any other known Trochoid. 



Formerly the West Indian Livonia pica, which has nine laterals, was quoted as the Trochoid with 

 the largest known number of these teeth (Fig. H, 17). A curious similarity between the radulae of 

 Venustatrochus and Livonia is that they both have a diamond-shaped base to the central. In Livonia, 

 however, the inner marginal is a massive hooked tooth and none of the teeth has the long, slender 

 serrated cusps of the Calliostomids. I do not consider that the resemblances noted between the radulae 

 of Venustatrochus and Livonia indicate close relationship, and certainly the shell features are discordant 

 in almost every particular. 



Venustatrochus georgianus n.sp., PI. X, fig. 68 



Shell large, very thin, conical with biangulate whorls, the lower one coincident with the suture and 

 forming the periphery of the last whorl; imperforate. Colour uniformly pale buff, with a greenish 

 iridescence showing through. Whorls eight (seven in holotype), including the protoconch, which is 

 small but projecting, slightly asymmetrical and consists of one unsculptured whorl somewhat immersed 

 at the tip with a dull, slightly malleated white surface. First two post-nuclear whorls with two spiral 

 threads above the peripheral carina and one below it ; third with two above and two below, fourth with 

 six above and three below, fifth with twelve above and ten below, penultimate with seventeen above and 

 sixteen below, and base with about forty threads. The suture is ledged by the uppermost thread, and 

 immediately below this on the later whorls there is a smooth, very slight concavity. The upper 

 surface of the whorls is crossed by retractively arcuate fine dense threads which render the spirals 

 minutely granular, but the basal spirals are smooth. Pillar plain, arcuate, much thickened above 

 and with a spreading callus which completely fills the umbilical area. Interior of aperture iridescent 

 and delicately spirally grooved. Outer lip thin and sharp. Operculum horny, circular, multispiral, 

 reddish brown. 



Diameter 40-0 mm. ; height 40-0 mm. (paratype). 



Diameter 32-0 mm. ; height 30-5 mm. (holotype). 



The smaller specimen is selected for the holotype, since the other has the protoconch missing and the 

 surface is eroded and encrusted. 



The species resembles in a general way Dall's Calliostoma platinum (1889 a , p. 343, pi. VII, fig. 2) from 



