I2 g DISCOVERY REPORTS 



approaches the New Zealand genus Zegalerus, which is of similar shape but has an even more simplified 



septum which is merely shallowly concave. 



Dentition. The paired marginals are plain as in trochiformis, but the main cusp on both the central 

 and laterals is much weaker. 



Height 7-5 mm.; diameter 12-5 mm. (holotype). 



Type locality. St. 160. Near Shag Rocks, West of South Georgia, 53 43' 40" S, 40° 57' W, 

 7 Feb. 1927, 177 m. 



St. 27. West Cumberland Bay, South Georgia, 3-3 miles S 44 E of Jason Lt., 15 Mar. 1926, no m. 



St. 140. Stromness Harbour to Larsen Point, South Georgia, 23 Dec. 1926, 122-136 m. 



St. 152. North of South Georgia, 53 51' S, 36 18' 30" W, 17 Jan. 1927, 245 m. 



St. 156. North of South Georgia, 53 51' S, 36 21' 30" W, 20 Jan. 1927, 200-236 m. 



St. 159. North of South Georgia, 53 52' 30" S, 36° 08' W, 21 Jan. 1927, 160 m. 



St. WS 33. Off south end of South Georgia, 54 59' S, 35 24' W, 21 Dec. 1926, 130 m. 



Family STRUTHIOLARIIDAE 



Genus Perissodonta Martens, 1878 



Type (monotypy): Struihiolaria mirabilis Smith, 1875, Recent, Kerguelen I. 



= Stnithiolarella Steinmann & Wilckens, 1908 



Type (o.d.): Struthiolaria ameghinoi v. Ihering, Oligocene, South America 



Members of the Struthiolariidae are restricted to southern lands, and their distribution can be 



accounted for only by an assumed former closer unity of these lands, either in the shape of the early 



Gondwanaland Continent of geologists, or by an enlarged Antarctica with northern radial extensions. 



The family was already well developed in the Upper Cretaceous of both Patagonia and New Zealand 



in the form of the gerontic genus Conchothyra, which shows relationship with the more widely distributed 



Aporrhaidae. 



In the Tertiary the Struthiolariidae were represented in Patagonia by Stnithiolarella (which seems to 

 equal the Recent Perissodonta), in New Zealand by a series of genera, Monalaria, Struthiolaria, 

 Callusaria and Pelicaria, and in southern Australia by Tylospira. 



The distribution of living members of the family is as follows : South Georgia {Perissodonta, one 

 species), Kerguelen Island {Perissodonta, one species), New Zealand {Struthiolaria and Pelicaria, one 

 species and a subspecies of each) and south-eastern Australia {Tylospira, one species). 



Marwick (1924), in his excellent monograph of the Struthiolariidae, referred the Kerguelen species 

 to Stnithiolarella and omitted reference to Marten's Perissodonta. This genus was again overlooked by 

 Finlay & Marwick (1937, p. 15), when a further valuable contribution was made to the phylogeny of the 



family. 



In the light of the adult characteristics of Perissodonta, revealed below for the first time by the 

 production of South Georgian examples with a fully grown, deeply sinused peristome, it seems certain 

 that the South American Oligocene? ornata (Sowerby) is identical with the recent P. georgiana. Both 

 species are presumed to be close to their respective genotypes. 



Unfortunately, there are no well-preserved apices in the several South Georgian examples, but 

 opercular and radular characters were noted. 



The operculum in P. georgiana is elongated leaf-shaped with an attenuated terminal nucleus and 

 a median sharply defined sulcus. In both Struthiolaria papulosa and Pelicaria vermis the operculum is 

 shaped like a reversed comma, for it is oval, but with a narrow hooked terminal nucleus. In papulosa 

 there are several radial planes, but these are ill defined (exaggerated in the text figure) compared with 



