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Description

Tropidomarga biangulata n.sp., Pl. V, fig. 5

 

Shell of moderate size, depressed-conical, with prominently shouldered spire-whorls and biangulate body-whorl, broadly umbilicated and sculptured with numerous spiral lirations crossed by dense retractive axial threads. Whorls six, including a low rounded protoconch of one smooth whorl. First post-nuclear whorl with two spiral lirae, second with six, body-whorl with twelve lirations between the suture and upper angulation, nine between the upper and lower angulations (about fifteen, angulations included) and about thirty on the base. They continue just over the edge of the umbilicus, but the remainder of the surface of the walls of the steep-sided umbilicus is plain. Umbilicus about one-fifth major diameter of the base. The suture is delicately crenulated by the axial threads. The angulations are broadly rounded and do not interrupt the sculpture.

 

Aperture rhomboidal, outer lip thin, strongly retractive in profile, inner lip inclined but moderately straight, slightly thickened medially and slightly encroaching upon the umbilicus, where it curves above and finally spreads a very thin nacreous glaze over the parietal wall. This glaze is so thin that it does not obliterate the spiral sculpture. Colour buff to very light brown above, creamy white on the base and iridescent within the aperture. Operculum horny, thin, yellowish brown circular and multispiral.

 

The biangulate whorls give the species a superficial resemblance to Strebel's Promargarita tropidophoroides.

 

Diameter 14.6 mm.; height 13.0 mm. (holotype).

 

Diameter 14.0 mm.; height 12.5 mm. (paratype).

 

 

TYPE LOCALITY. St. 159. North-east of Cumberland Bay, South Georgia, 53° 52' 30" S, 36° 08' W, 21 Jan. 1927, 160 m.

 

St. 170. Off Cape Bowles, Clarence I., 61° 25' 30” S, 53° 46' W, 23 Feb. 1927, 342 m.

 

 

DENTITION Fig. G, 6, p. 189 8+4+1+4+8. Very similar to that of ‘Margarita striata’=Pupillaria cinerea Couth (Thiele, 1891, pl. 25, fig. 9) from Gulf of St Lawrence. Both have the central tooth with a circular base and laterals of the same form except that each is cut away on its inner side.

 

Thiele’s drawing of the radula of 'striata' shows a vestigial fifth lateral and a very weak innermost marginal. In biangulata there is no trace of a fifth lateral, and the innermost marginal is the largest of that series. But it is not disproportionately large as in the Calliostomids.

 

 

ANIMAL. There are six pairs of epipodial tentacles-the sixth from the front long and slender, twice the length of the other five, which are of approximately equal size. Cephalic tentacles moderately long, blunt and flattened. Eye at the end of a short stalk, lying at the outer side of the base of each tentacle. The stalk is confluent with a short veil which crosses the base of a tentacle but does not join up with its opposite member across the head. Head narrow in front and produced into a wrinkled proboscis, with a frilled edge. Epipodial fringe continued to the area in front of the head in an elaborate series of folds.”

 

 

(Powell, 1951:101-102)

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Creative Commons Attribution Non Commercial 3.0 (CC BY-NC 3.0)

© National Museum of Natural History, Smithsonian Institution

Source: Antarctic Invertebrates Website (NMNH)

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