SYSTEMATIC 129 



the deep sulcus in georgiana. In vermis there are no radial planes, just a slight strengthening rib along 

 the lower inner margin (Fig. N, 120, 121, p. 196). 



I have not seen the operculum of Perissodonta mirabilis, but Smith's figure (1877, pi. fig. 3«) shows 

 a prominent hook, but this may have been due to warping of the operculum in drying and the breaking 

 away of the lower outer margin. However, a significant similarity to the georgiaiia operculum is the 

 median sulcus which is clearly shown in Smith's figure. 



Some interesting points arise from study of the dentition of three of the Recent genera : Stmthiolaria, 

 Pelicaria and Perissodonta (Fig. I, 35-37, p. 191)- First, Stmthiolaria and Pelicaria, as one would expect, 

 show closer alliance to each other than either do to Perissodonta, which stands apart by the duplication 

 of its marginals to four or five teeth and the erect, narrowly triangular shape of the laterals as opposed to 

 the rectangular outline of these teeth in both Stmthiolaria and Pelicaria. The triangular-shaped lateral of 

 Perissodonta approaches that of Aporrhais, and this feature, coupled with that of the deep labial sinus, 

 makes the derivation of the Struthiolariidae from the Aporrhaidae a very reasonable assumption. 



Duplication of marginals is evidently an archaic stable feature of Perissodonta, for it occurs in the 

 four examples of georgiana studied, one of which was prepared by Mr John Morton of Auckland 

 University College, who first drew my attention to this fact. Fischer (1884, Text-fig. 442) gives 

 a curious rendering of the radula of the Kerguelen P. mirabilis (as Stmthiolaria costulata Martens), in 

 which the laterals and marginals are drawn in an unnaturally erect position, but the essential points, 

 five pairs of marginals and tall narrow laterals, are shown. 



In Perissodonta georgiana all the teeth are denticulate to some degree. The central has blunt, irregular 

 denticles that vary greatly even from tooth to tooth in the same radula. The lateral has a broadly rounded 

 terminal cusp followed by a series of denticles, the innermost marginal is intermediate between the 

 lateral and the slender marginals and bears ten to twelve small but regular denticles, while the outer 

 marginals have only weak serrations towards their blunt, narrowly rounded extremities. 



Although both Stmthiolaria and Pelicaria have normal taenioglossid paired marginals and almost 

 identical rectangular laterals, the central tooth is distinctive for each genus. That of Stmthiolaria is 

 square-based with a multiserrate, broadly triangular cutting edge that does not extend to half the depth 

 of the base. That of Pelicaria is oblong, more broad than deep, but has a very large narrowly triangular 

 serrated cusp that extends far below the base. In Pelicaria both marginals are plain, but in Stmthiolaria 

 the inner one is weakly denticulate on its lower inner margin. Unfortunately, the dentition of Tylospira 

 is not known. 



Perissodonta georgiana Strebel, PI. VIII, figs. 40-42. 



Perissodonta mirabilis georgiana Strebel, 1908, p. 46, pi. 3, figs. 33a, b, c. 



Type locality. Cumberland Bay, South Georgia, 252-310 m. 



St. 1941. Leith Harbour, South Georgia, 29 Dec. 1936, 38 m. 



St. WS 33. Off Southern end of South Georgia, 54 59' S, 35 24 W, 2 Dec. 1926, 130 m. 



St. WS 62. Wilson Harbour, South Georgia, 19 Jan. 1927, 26-83 m. 



Dentition. Fig. I, 35, p. 191, georgiana; Fig. I, 36, Stmthiolaria papulosa, Takapuna, Auckland, 

 New Zealand; Fig. I, 37, Pelicaria vermis, Auckland Harbour, New Zealand. 



Operculum. Fig. N, 120, georgiana; Fig. N, 122, Stmthiolaria papulosa, Fig. N, 121, p. 196, 

 Pelicaria vermis. 



