489 



TROCHID^. 



The Top-shells constitute a numerous family, remarkable 

 for exquisite beauty of form and colour, both of shell and 

 animal. Some of our most elegant British univalves belong 

 to this tribe, and many of the most ornamental exotic 

 shells. Their animals hav^e all pedunculated eyes ; heads 

 terminating in a muzzle, and often ornamented with head- 

 lobes ; side-lobes greatly developed, and ornamented with 

 lappets and lateral cirrhi ; a tail bearing a spiral oper- 

 culum, sometimes horny, sometimes shelly ; a single bran- 

 chial plume, and a dental system remarkable for the 

 number of accessory lateral denticles. The shells vary 

 much in form, ornament, and structure, some being discoid, 

 whilst others have greatly produced spires ; though, in all, 

 the spire forms a very considerable portion of the entire 

 shell. Their orifices are entire, but vary much in out- 

 line. 



In formations even of palseozoic age we find the shells of 

 Mollusca of this family often resembling closely, in general 

 outline, existing forms. The so-called EuompJiali, Cirrhi^ 

 and the greater number of Pleurotomaria.^ appear to have 

 been aUies of Delpliinula, Turho^ and Trochus. 



TROCHUS. 



Shell very variable in form, sculpture and strength, 

 conical, subglobose or subdepressed, texture partly nacreous, 



VOL. II. 3 R 



