10 
THE VOYAGE OF H.M.S. CHALLENGER. 
The operculum is very delicate, glassy, and transparent. It is fixed by a portion 
of its surface to the posterior face of the ventral lobe of the foot. 
The animal is twisted like the shell which it completely fills, and into which it may 
be completely retracted. The margin of the mantle bears, on the right-hand side, and 
somewhat ventrally, a long extensile appendage. The posterior lobe of the foot, which 
bears the operculum and is topographically ventral, is hollowed out on the middle of its 
free margin. The fins do not exhibit, towards their distal extremity, the area without 
muscular fibres which is usually to be observed in the genus Clio } 
As regards the systematic relations of the genera and species, the family Limacinidse 
is still but imperfectly understood. This is in part doubtless due to the small size of the 
animals which form the family. They have hitherto been but rarely studied, and even 
in special works on Pteropods are often slurred over, as for instance in the memoirs of 
Quoy and Gaimard and of Rang. In the same way Troschel and Gegenbaur in their 
studies on the Pteropods of the Mediterranean have not discussed a single member of 
this family, and we may also note that Pfeffer, who has published an important 
description of the Thecosomata in the Hamburg Museum, has quite overlooked the 
Limacinidse. 
The investigation of the numerous specimens of this family which were collected on 
the Challenger Expedition has enabled me to make an almost complete study of the 
entire family. The results of my investigation I therefore proceed to submit. 
If one considers the living species alone, one finds in the literature of the subject 
that there are no less than thirty-six different specific names applied to forms referred to 
this family. In this number I do not include, be it understood, the manuscript species, 
or those which have been simply recorded without description or figure — Limacina 
carinata, Jeffreys , 2 Spirialis diversa, Monterosato , 3 Spiricdis contorta, Monterosato . 4 
These I evidently could not take into account. 
Since the work of Souleyet , 5 Boas is the only naturalist who has attempted to make 
a synthetic study of this group . 6 
From the researches of these authors it may be concluded that there are now seven 
species adequately enough known by their shell, operculum, and anatomy to leave no 
doubt as their systematic position. These species are the following, and in citing them 
I shall retain the original generic titles, omitting for the present the discussion of their 
proper generic distribution. 
1 Boas considers this space as corresponding to the hollow which separates the small tentacle-like lobe of the fin 
of some species of Limacina and Clio of the subgenus Creseis, from the margin of this fin (Spolia atlantica, p. 182, 
pi. v. figs. 70-79). 
2 The French Deep-Sea Exploration in the Bay of Biscay, Rep. Brit. Assoc., 1880, p. 387. 
3 Nuova rivista delle conchiglie Mediterranee, p. 50. 
4 Ibid., p. 50. 
5 Histoire naturelle des Mollusques Pteropodes. 
6 Spolia atlantica, pp. 38-50. 
