lO 



eye by a somewhat larger size. To the smallness of the animals it must be attributed tliat 

 before Boas and Pelseneer only a few attempts have been made, to bring some order in the 

 confusion, which also reigned here. 



SouLEYET ^) distinguished two genera: Limacina without operculum, and Spirialis with 

 operculum. Since, however, an opercuhmi has been found -) in Limacina helicina, the only 

 representative of the genus Limacina, as Souleyet understood it, now the difference between 

 the two genera cannot exist any longer. Boas *) brought all the species of the family together 

 to the single genus Limacina, but Pelseneer's researches *) have proved that there are two 

 very distinct genera : Limacina and Pcraclis. 



The study of the collection, brought home by the Challenger, has given occasion to 

 Pelseneer to distinguish °) ten species. Two of them, however, must be considered rather 

 doubtful, as the animals have never been found as yet, and only the empty shells were brought 

 up from the bottom of the sea {Limacina triacaniha and Limacina hclicoidcs). So we cannot 

 state with certainty, whether we have to do with "Pteropoda". 



As the Siboga Expedition has onlj- caught three species of Limacina, and as of the 

 other species I could only study Limacina hclicina, I have not been able to observe the specifïc 

 differences, given by Pelseneer. The three species of the Siboga-material, however, are clearly 

 distinct from each other. 



Of the genus Peraclis two species have been known up to now, though of Pcraclis 

 bispinosa *) only the empty shells have been found. The Siboga Expedition has added two new 

 species, and a variety of an already known one. 



With a few words I must draw the attention to the well-known fa et, that the shell of the 

 Limacinidae is left-coiled, while the animals themselves possess the organisation of right-handed 

 Gastropoda. The shell is in this case ultra-dextral which, as one knows, also occurs in some 

 species of the genus Ampullaria. The spira of a right-coiled shell may become more and more 

 flattened, then the shell may be rolled up in one plane, and finally it appears in the place 

 where originally the umbilicus has been, which now takes the place of the spira. The operculum 

 does not take any part in this process, consequently it retains the direction of its coils. 



According to Souleyet '), the operculum of the Limacinidae is right-handed, except in 

 one case, where it is figured in situ in Limacina bulimoides "). 



In Sars's paper ^) too, the operculum of Limacina helicina, '^Spirialis'' balea and 



1) Hist. nat. d. MoU. Ptévop. p. 60 — 65. 



2) Saks, Mollusca regionis aicticae Xorvegiae, Chiistiania, 1S78, p. 32S, pi. 29, iig. ie. 



3) Spolia atlantica, p. 39. 



4) Chall. Rep. LXV, p. 9—15. 



5) Ibid., p. 17—37- 



6) Ibid., p. 36. 



7) Voyage de la Bouite, MoUusques, pi. XIII. 



8) Op cit. pi. XIII, fig. 36. 



9) Mollusca regionis arcticae Xorvegiae, pi. 29, figs. \e, 2(/, 3«'. 



