74 pupiD^. 



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 continuall}'^ 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 calcareous fila- 

 ment, one of whose ends is affixed to the columella. This fila- 

 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 elasticitj^ 

 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 inquir3^" 



The following arrangement of the subgenera or sections of 

 Clausilia is essentially that of 0. 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. 

 G. Tristensis, Leach, and G. 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, 1870. (Filholia, Bourg., 1877.) Shell 

 very large, with pear-shaped aperture, continuous peristome, and 

 columella with a basal lamella. Bulimiis laevolongus, Boubee. 

 Fossil. Lower Tertiary ; Europe. 



