22 
THE VOYAGE OF H.M.S. CHALLENGER. 
South Alaska (35° 50' N.) (Dali) ; Arctic Ocean : Point Barrow (surface temperature, 
40 o, 2 F. to 42°) (Dali); 1 Aleutian Islands (Krause). 
Observations. — I. As in all other species of Limacina there is an operculum, but it is 
caducous in the older specimens. I emphasise the presence of this operculum, because 
recent works, such for instance as Claus’s Textbook, still characterise Limacina (s. str. 
type Limacina helicina) by the absence of this structure. In 1878 Sars definitely 
demonstrated its existence. 
The caducous character of the structure is no isolated fact, for we know not only 
genera [Fleur otoma, Voluta, &c.) in which certain species have, and others have not, an 
operculum, but also species in which it is sometimes present, sometimes absent, according 
to the individual ( Volutharpa ampullacea, Middendorf). 
II. I consider Limacina pacifica, Dali, as identical with Limacina helicina. In fact, 
according to Dali and Krause, this form differs from Limacina helicina only in having 
more whorls on its spire, and in the absence of an operculum. But these are precisely 
the characters of the adult Limacina helicina, with which Limacina pacifica is therefore 
identical. 
# 4. LAmacina antarctico, Woodward (PI. I. figs. 3, 4). 
1856. Limacina antarctica, Woodward, A Manual of the Mollusca, p. 207, nomen tantum, 
pi. xiv. fig. 4. 
Characters and Description. — Subdiscoidal, flattened shell, with a spire very slightly 
elevated, rather depressed. Six whorls, ribbed superiorly like those of Limacina helicina, 
and, in spite of the slight elevation of the spire, quite distinct, and separated by a w r ell- 
defined suture. The whorls are rounded externally and interiorly, the last is much 
expanded. Large mouth, broader than high, rounded off externally, and not prolonged 
into an angle. Arched columella. Large umbilicus, not surrounded by a keel as in 
Limacina helicina. Transverse striae, perpendicular to the axis, arched, regular, and 
equidistant. 
Dimensions. — Maximum diameter 4 to 5 mm.; the height equal to about half the 
diameter. 
Operculum unknown, probably like that of Limacina helicina, and caducous in the 
adult specimens, which alone have been examined. 
Animal very like that of the preceding species. The anterior (dorsal) marginal lobe 
of the fin is very small, though perhaps somewhat contracted in the only specimen in 
which I was able to examine it. The posterior lobe of the foot is markedly hollowed out 
on its free margin. Tentacles like those of Limacina helicina ; the left rudimentary, 
the right very long, situated in a sheath, and further forward than the left. 
1 Report on the international North Polar Expedition to Point Barrow, 1885, Report on the Mollusks, p. 
