SYSTEMATIC 105 



Type locality. Port Rosario and Portland Bay, Patagonia. 



St. 1321. From 4 miles S 72 W to 5-6 miles S 75° W of East Tussac Rock, Cockburn Channel, Tierra del 

 Fuego, 16 Mar. 1934, 66 m. 



One example of this attractive little rose-coloured, finely striated species; diameter 4-75 mm., 

 height 4-0 mm. The operculum is calcareous and spirally grooved on its outer surface for 1^ whorls, 

 enclosing a roughened central area. 



I have followed Vokes (1939, p. 179) in preferring Homalopoma to the better known name Leptothyra. 



Genus Leptocollonia n.g. 

 Type: Leptocollonia thielei n.sp. 



This new genus is provided for a southern group of Homalopoma-like (Leptothyra) shells which differ 

 in being umbilicate, relatively thin-shelled, colourless, and in having a distinctive operculum, which is 

 concave externally and deeply spirally channelled. The radula of Leptocollonia is very similar to that ot 

 Homalopoma, the chief difference being in the marginals, which are plain in the former and serrated in 

 the latter. 



Thiele's Leptothyra innocens (1912, p. 192, pi. n, fig. 24) from Gauss Station, Antarctic, is a Lepto- 

 collonia, and the operculum of Leptothyra sp. (Thiele, loc. cit.), also from Gauss Station, is so similar 

 to that of thielei n.sp. that it probably originated from this same species. 



Leptocollonia thielei n.sp., PI. V, fig. 9 



Shell small, depressed-turbinate, perforate, dull yellowish buff, sculptured with narrow prominently 

 raised spiral ridges. Whorls rounded, four, including the protoconch which is apparently one slightly 

 convex and smooth whorl (protoconch eroded in all examples). First post-nuclear whorl with two spiral 

 ridges, penultimate with from six to eight (seven plus an incipient eighth in holotype), body-whorl with 

 fifteen, last two bordering the umbilicus, weak. The spirals are rounded but narrow and prominently 

 raised with interspaces of from three to four times their width. The whole surface is crowded with 

 delicate axial growth striae. Umbilicus narrow and deep, about one-ninth the major diameter, but 

 half-obscured by the thin refiexed outer edge of the columellar callus. Peristome continued across the 

 parietal wall by a thin glaze. Outer-basal lip thin and corrugated by the terminal points of the external 

 spiral sculpture. The operculum is calcareous and multispiral of about seven whorls. Externally dull 

 white, concave and deeply spirally channelled. Internally smooth and convex with a yellowish chitinous 

 layer. The edge is thick, bevelled, with a median groove. 



Diameter 9-0 mm. ; height 7-5 mm. (holotype). 



Dentition. Fig. G, 13, p. 189. The radulae of these small Turbinids are most difficult to interpret 

 on account of the minute size of the teeth and the complicated manner in which the bases overlap. My 

 conclusions were made prior to reference to the results arrived at by Troschel & Thiele (1878, p. 213, 

 pi. 22, fig. 7) and Tryon& Pilsbry (1888, pi. 60, fig. 73). The Leptocollonia thielei radula (Fig. G, 13) 

 shows a marked resemblance to Pilsbry 's figure of that of Homalopoma carpenteri (Fig. G, 14), and is 

 almost totally at variance with Troschel and Thiele's interpretation based upon ' coccineus Deshayes', 

 the same species. The ' double-decked ' appearance of the central tooth is not paralleled in any other 

 known group. In both Homalopoma and Leptocollonia this tooth has a projecting plate above as well as 

 below, and lateral wings form a broad arc across the median area. The central appears to be function- 

 less, for it bears no cusps, just an irregular thickening at the crest of the median arc. The laterals, five 

 in number, are long, excavated on the inner side to accommodate the lateral wings of the central, and 

 produced on the outer side. These features are well developed in four of the laterals, but not in the fifth. 



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