LACUNA. 55 



LACUNA, TuRTON. 



Shell turbinate, solid or thin, obliquely conoidal or sub- 

 globular, spire short or produced, surface smooth (in the 

 British species) protected by an epidermis ; mouth ample, 

 rounded, peritreme entire, not continuous, outer lip sharp- 

 edged, columellar lip expanded, grooved, umbilicated. 

 Operculum semicircular, corneous, of few rapidly increasing 

 whorls, the spiral nucleus lateral and subterminal. 



Animal having a muzzle-shaped head, with two long 

 tentacula, bearing eyes or bulgings at their external bases. 

 No neck-lobes ; operculigerous lobe expanded or winged 

 laterally and furnished behind with two filamentary pro- 

 cesses, more or less developed, but sometimes nearly ob- 

 solete. Foot rounded at both extremities, contracted at 

 the sides, centrally grooved. Branchial plume single. 

 Male organ long, thick, compressed, placed near the right 

 tentacle. Lateral elements of the tongue heterogeneous ; 

 two of the uncini as well as the median denticle with 

 incurved (five) denticulated apices. 



The mollusks of this genus, which was instituted by 

 Turton, and is one of the most natural groups of its order, 

 were formerly confounded with Nat'ica^ and, even now, 

 are frequently associated by conchological authors with 

 genera to which they have no near affinity. They live 

 upon sea-weed, usually inhabiting the fronds of Laminaria 

 and Fucus., in the zone of which the former plant is 

 characteristic ; more rarely ranging to greater depths. The 

 peculiarities of the animal were first described by Philippi. 



Loven states that their ootheca are vermiform, thick, 

 and curved in a semicircle. 



The species are mostly of northern seas. Their geological 

 range is not yet clearly understood. 



