NEKITID.E. TURB1NID.E; TROCIIU8. 381 



is sometimes completely filled up by the exudation of solid mat- 

 ter. Some modern Conchologists place the Magilus amongst the 

 Buccinidae. 



996. We come now to the NERITID^E, which are furnished 

 with a thick and somewhat globose shell, with a small spire, and 

 an entire aperture, which, however, is narrowed by the broad 

 plates which are directed inwards from the margins, and the 

 edges of which are frequently notched. The inner walls of the 

 spire are removed as the animal grows from them, so that the 

 interior of the shell forms a simple cavity. The animal has 

 long slender tentacles, near which the eyes are situated on short 

 foot-stalks ; the lingual teeth are very numerous. The opercu- 

 lum is shelly, partially spiral, and furnished with an appendage 

 on one side near the nucleus. Most of the Neritidse are marine, 

 and they are almost confined to tropical climates. The genus 

 Neritina is fluviatile in its habits ; one species inhabits Britain. 



997. The TURBINID^E, or Top-shells, are nearly allied to the 

 Neritidse, but differ from them in several important particulars. 

 The shell is spiral, and usually more or less conical in form ; and 

 the operculum, which is always spiral in its structure, is destitute 

 of the peculiar appendages of that of the Neritidse. The inte- 

 rior of the shell is pearly. The head is generally furnished with 

 pectinated lobes ; and the sides present several long filaments or 

 cirri. Of the typical genus Turbo, some of the larger tropical 

 species are often seen with the shelly matter partially or entirely 

 removed, so as to expose the inner pearly stratum. This oper- 

 ation is also performed upon the larger shells of the genus Tro- 

 chus, some of the smaller species of which are exceedingly 

 abundant on our coasts. In the Trochus the form of the shell 

 is pyramidal, the sides being usually nearly straight, and the 

 aperture situated at one side of the flattened base. The shells 

 of the small British species are the favourite habitation of young 

 Hermit Crabs. Amongst the exotic species we may especially 

 notice the Trochus agglutinans, a native of the West Indies 

 which derives its name from its singular habit of glueing to its 

 shell small pieces of stone, coral, shell, &c. This seems to answer 

 the purpose of strengthening its shell, which is thin and brittle. 



