SYSTEMATIC 101 



Genus Tropidomarga n.g. 

 Type: Tropidomarga biangulata n.sp. 



The type of this new genus bears a superficial resemblance to Minolta, but the dentition shows close 

 alliance with the boreal ' Pupillaria' cinerea (Couthouy). Both differ from all other Margarites-like 

 species (of which the dentition is known to me) in the form of the basal plates of the central and laterals. 

 The central has a large circular base, and the laterals are similar except that they are cut away on the 

 inner side. 



The type of Pupillaria (Trochus pupillus Gould, California) is tall, conical, solid, angulate at the 

 periphery, narrowly umbilicate and sculptured with narrow spiral cords. The species cinerea (boreal) 

 is relatively large and thin with a rounded periphery, surface sculptured with narrow, distinct, sharply 

 raised spiral cords and has a deep narrowly open umbilicus about one-eight the major diameter of the base. 



The type of Margarites {Helix margarita Montagu = Turbo helicinus Phipps) is depressed turbinate, 

 umbilicate and smooth surfaced. 



Although there is apparently a relationship between the antarctic Tropidomarga and the boreal 

 cinerea, the latter does not conform to the genotypes of either Margarites or Pupillaria, or any other 

 available genus. 



For the antarctic species described below, I propose therefore the new generic name Tropidomarga. 



This new genus has a conical shell with strongly shouldered spire whorls and a prominently bi- 

 angulate body-whorl. The surface is sculptured with thin spiral lirations crossed by finer dense axial 

 threads. At the suture the axials are thickened to form a crenulated submargining. The umbilicus is 

 deep with straight-sided walls and has a width of about one-fifth that of the major diameter of the shell. 



Tropidomarga biangulata n.sp., PI. 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 as 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., 6i° 25' 30" S, 53 46' W, 23 Feb. 1927, 342 m. 



