16 
THE VOYAGE OF H.M.S. CHALLENGER. 
umbilicus vary according to the species. Operculum semilunar, with a right-handed 
spiral of a few whorls. 
Animal with an indistinctly defined head, which is only marked externally (1) by the 
lips on the border of the mouth and (2) by the tentacles. 
1. Lips : two dorso- ventral folds on the cephalic surface of the fins, united dorsally, 
diverging ventrally, where they are continued by a fold of the cephalic surface of the 
fins, and extend laterally to the edge of the fins. They thus enclose a ciliated area 
which plays an important part in alimentation. 1 The mouth, split dorso- ventrally, is 
situated between these lips, in the angle formed by their union. 
2. Tentacles, asymmetrical, the left always less developed and further back than the 
right. The latter is very long and retractile into a sheath. The tentacles thus exhibit 
absolutely the same form as those of the Cavoliniidse. Souleyet 2 noted that in Limacina 
helicina the right tentacle seemed to be situated in a sheath, and 3 that in his “ Spirialis ” 
the minuteness of the organs did not permit him to observe whether this was again true. 
I have been able to convince myself that this sheath exists, not only in Limacina 
helicina, but in all the small species in which I have been able to study the animal, viz., 
Limacina injlata, Limacina lesueuri, Limacina australis, Limacina trochiformis. 
Fins elongated, enlarged, truncated at their free end. In certain species — Limacina 
helicina (where the structure has been noted by P. J. van Beneden under the name of 
tentacles), Limacina antarcticci and Limacina australis (where it was equally distinct) — 
the fins exhibit, towards the middle of their dorsal margin, a small narrow projecting 
lobe of a special structure. A similar structure exists in Clio in the subgenus 
Creseis. I have assured myself that in Limacina injlata, Limacina lesueuri, and 
Limacina trochiformis this small lobe is not present, and Boas vouches for its absence 
in Limacina bulimoides. In the other species the animal has not yet been examined. 
I cannot attach any great systematic importance to the presence or absence of this 
minute lobe, or regard it as furnishing basis for generic or subgeneric distinction, for 
otherwise the entire organisation is so absolutely analogous in all the species of Limacina, 
and the lobe is present in Limacina australis, and absent in Limacina retroversa, 
species so closely allied that some authorities have doubted whether they were really 
distinct. 
On turning to the table of species of Limacinidse, it will be seen that there are only ten 
species which belong to the genus Limacina properly so called. Of these, seven are well 
known by their shell, their animal, and their operculum, while the other three are 
sufficiently well known to enable one to judge with some certainty in regard to their 
systematic position. 
1 See Boas, Spolia atlantica, p. 191. — An identical disposition is found in the Cavoliniidas. 
2 Histoire naturelle des Mollusques Pteropodes, p. 60. 
3 Voyage de la Bonite, Zoologie, t. ii. p. 209. 
