74 PUPIDA. 
univalves from the intrusion of enemies; hence the apertures 
are sometimes contracted and provided with numerous folds and 
teeth. Other mollusca have a calcareous operculum permanently 
formed, which increases in thickness, and enlarges on a depressed 
spiral plane, as the opening of the shell extends with the growth 
of the animal, thus continually assimilating to its size, and when 
the animal retreats, excluding it completely from all external 
intrusion. In the Clausilia nature has continued the protection 
afforded by means of contractions and folds, and also added an 
opercular appendage. The inhabitant of the Clausilia, when 
nearly full-grown, secretes a thread-like elastic caleareous fila- 
ment, one of whose ends is aflixed to the columella. This tila- 
ment makes half a spiral turn round the columella, insinuating 
between its folds. When the animal finishes its shell and com- 
pletes the aperture, it secretes, at the unattached end of the 
filament, a spoon-shaped calcareous lamina conforming at its 
margin to the contour of the aperture. The lamina is somewhat 
smaller than this, and its margin is rounded. Its adhesion to 
an elastic filament enables the animal to push it, when it comes 
out of the shell, against the columella; and the same elasticity 
closes it on the inhabitant retreating, thus securing it from 
intruding enemies. Thus, then, this valve may be compared to 
a door provided with an elastic spring. The elasticity of the 
filament may be restored to its full power (in the empty shell) 
by sometimes immersing it in water, as I have ascertained in a 
section made with a view to this inquiry.” 
The following arrangement of the subgenera or sections of 
Clausilia is essentially that of O. Bottger, who has devoted much 
time and attention to the study of the systematic relationship of 
the species. 
BALEA, Prid., 1824. (Baleastra, Pfr., 1855. Tristania, Bottg., 
1878. Pseudobalea, Shuttl., 1854.) Shell thin, spiral, turricu- 
lated, sinistral; aperture rounded in front, angular behind, 
without lunule; columella simple or uniplicate; no clausilium. 
C. Trisiensis, Leach, and C. perversa, L. (c, 20). Europe, 
Tristan d’Acunha, New Zealand. The want of posterior lunule 
or bay of the aperture and of a clausilium, makes this an 
aberrant form, and it has sometimes been considered a distinct 
genus; it is united to the typical Clausilia, however, through 
the group Alopia, Adams, in which the clausilium is sometimes 
absent, sometimes more or less developed. 
TEMESA, H. and A. Adams, 1855. Columella plicate at the 
base. B. Clausilioides, Reeve (c, 19). Peru. 
DACTYLIUS, Sandberger, 187C. (Filholia, Bourg., 1877.) Shell 
very large, with pear-shaped aperture, continuous peristome, and 
columella with a basal lamella. Bulimus levolongus, Boubée. 
Fossil. Lower Tertiary ; Europe. 
