MOLLUSCA. 489 



Section 1. Marine. 



This includes the genera Nerita and Natica. 



Section 2. Fluviatile. 



This contains only the genus Neritina, including Nerita 

 JluviatUis of Linnaeus. 



Family 3. Trochusidcc. Aperture of the shell subqua- 

 drangular. 



All the genera, including Trochus, Solarium, and Pyra- 

 midella, are marine. The cloak on each side is usually or- 

 namented with three filaments. 



2. Aperture of the shell exposed. — The foot destitute of 

 a lid. Marine. 



52. Janthina. Foot with an adhering spongy body. 

 In this genus, represented by the Helix janthina of Lin- 



N.EUS, the spongy body is capable of changing its dimen- 

 sions, and enabhng the animal to sink or rise in the water 

 at pleasure. When irritated, it ejects a purple fluid from 

 the cellular margin of the cloak above the gills, not unlike 

 the Aplysia. This species was added to the British Fauna, 

 by the late Miss Hutchins. 



53. Velutina. Foot simple. 



This genus was formed by us for the reception of the 

 Bulla velutina of Muller {Zool. Dan. tab. ci. f. 1, 2, 3, 4), 

 the Helix laevigata of British writers. 



The foot is destitute of lid or appendage, and is broad 

 before, and pointed behind. The tentacula are two in num- 

 ber, short and filiform, with eyes at their external base. 

 The head is broad and short. In addition to these charac- 

 ters given by Muller, we have been enabled to add the 

 following, from a specimen, somewhat altered, which was 

 found in the stomach of a cod-fish. The animal adheres to 

 the shell by two linear muscles, one on each side the cloak. 

 The branchial cavity is towards the left side. The tongue 

 is spinous, narrow, with its free extremity spiral. Eyes 



