72 PROCEEDINGS OF THE MALACOLOGICAL SOCIETY. 



or two on the columella, and from three to five on the palatal margin, 

 some of these being strongly developed. 



Genus Vertigo, Muller (sensu lato), 

 also a genus of numerous minute forms, is of world-wide 

 distribution. Many of the species have the mouth very much 

 obstructed by folds and teeth, although in a certain number, such 

 as V. edentiila, Drap., and F. minutissima, Hartm., two British 

 species, these barriers are absent. F. ovata, Say, a North American 

 species, has generally six lamellae : two parietal, two columellar, and 

 two palatal. 



It only remains for us to consider the 



Genus Clausilia, Draparnaud. 

 A large genus of wide distribution, being found in Europe, Asia, 

 Northern Africa, South America, and Porto Rico. A great number 

 of sections or subgenera have been established, some of doubtful 

 value. The aperture is comparatively small, usually pear-shaped, 

 provided with two spiral, entering lamellae — usually on the parietal 

 wall, the lower sometimes on the outer lip — continued internally 

 as far as the seat of attachment of the pedicle of the clausilium ; 

 the upper follows the spiral convolution of the columella, and becomes 

 the columellar fold, a second fold further back is known as the 

 sub-columellar fold ; these two folds form a long, flexuous groove, 

 slightly dilated towards the aperture, but contracted further down. 

 A curved, flexuous, tongue-shaped, elastic plate, known as the 

 clausilium, characterizes and gives its name to the genus ; higher up 

 it becomes contracted into a narrow, twisted pedicle, its distal 

 extremity attached to the inner shell-wall, between the distal 

 extremities of the columellar and sub-columellar folds, the groove 

 between these, lower down, receiving the clausilium as it is pushed 

 to one side by the animal's extrusion. In addition there are a number 

 of palatal plicae behind the aperture and usually showing through the 

 shell-wall. In some species two of these plicae have the posterior 

 extremities curved and approximating, ultimately uniting and 

 forming the so-called lunella. While the animal is retracted within 

 its shell the elastic pedicle causes the clausilium to rest against the 

 sub-columellar fold on the inner side and against the shorter palatal 

 plicae or the lunella, when present, on the outer side, the anterior 

 angle of its inner margin slightly projecting inwardly over the sub- 

 columellar fold, an arrangement which effectively prevents the 

 clausilium being forced to one side from without, thus securing the 

 animal against intruding enemies. During extrusion of the animal 

 the clausilium is pushed sideways into the groove between the 

 columellar and sub-columellar folds, only its anterior portion being 

 pressed slightly forward at the dilated part of the groove. The 

 clausilium may, therefore, be regarded to act as a sliding door, 



