REPORT OX THE PTEROPODA. 
21 
unaccompanied by any figure, does not, however, admit of certain decision on this 
point. 
The fact that this species has only been found in the deep-sea deposits, and that 
over a wide area, but has never been collected alive at the surface, raises the question 
whether it be not really extinct. The remains of fossil Elasmobranchs and Cetaceans 
found at the bottom of modern seas make this hypothesis more plausible. 
Although neither the animal nor the operculum of Limacina triacantha have been 
as yet observed, there cannot be any doubt that the form in question is a Pteropod. It 
is indeed difficult to decide, in the absence of the animal, that an empty shell, twisted in 
a left-handed spiral, really belongs to the Thecosomatous Pteropods and the group 
Limacinidse, and not to some Gastropodous group, but in the present case there can hardly 
be any hesitation, as is shown by the following characters 
1. The constancy of the left-handed spiral of the shell, which is observed in all the 
specimens, shows that we have not to deal with an abnormal left-handed example of a 
right-handed Gastropod. 
2. The great breadth of the transverse diameter is not what is normally found in 
left-handed Gastropods, which usually exhibit an elongated spiral. 
3. The thinness of the test is also suggestive. 
4. The predominance of Thecosomatous Pteropod shells in the sediments from which 
the present species was dredged is in itself an argument. 
5. The numerous resemblances between the species and Limacina inflata suggest a 
close affinity. 
3. Limacina helicina (Phipps). 
1774. Clio helicina, Phipps, A voyage towards the Xorth Pole, p. 195. 
1774. Clione helicina, Pallas, Spicilegia zoologica, fasc. x. p. 38. 
1780. Arcjonauta arctica, Fabricius, Fauna grcenlandiea, p. 386. 
1819. Limacina helicialis, Lamarck, Histoire naturelle des animaux sans vertebres, t. vi. p. 291. 
1824. Spiratella limacina, de Blainville, Diet. d. Sci. Xat., t. xxxii. p. 284. 
1832. Spiratella arctica, Deshayes, Encyclopedie methodique, Vers, t. iii. p. 138. 
1841. Limacina arctica, Moller, Bemaerkninger til slaegten Limacina, Krpyer, Xat. Hist. Tidsskr., 
1 Raekke, Bd. iii. p. 488. 
1852. Limacina helicina, Souleyet, Histoire naturelle des Mollusques Pteropodes, p. 61. 
1872. Limacina pacifica, Dali, Description of sixty new forms of Molluscs from the West Coasts 
of Xorth America and the Xorth Pacific Ocean, Amer. Journ. of 
Conch., vol. vii. p. 138. 
For description and figures, I refer to Sars, Mollusca regionis arctic® Norvegise (1878), 
p. 328, pi. 29, fig. 1. 
Habitat. — The area of distribution is similar to that of Clione limacina. — Davis Strait ; 
Hudson Strait ; Greenland ; Iceland ; Jan Meyen Island ; Southern Norway ; White Sea 
(Wagner); Spitzbergen ; Nova Zembla ; Sea of Okhotsk; North Pacific: Monterey, 
