122 GENUS XXVII. TROCIIUS. 



Trochi vary as much in their general form as in the mouth : 

 some are conic, as before mentioned ; others are very de- 

 }3ressed, as T. perspectivus and Solaris ; some approach the 

 globular form, as Labio ; many species have an irregular but 

 conspicuous tooth near the extremity of the pillar, as T. Pha- 

 raonis, Labio, &c. ; others have three or four small teeth, as 

 T. virgatm ; some of the last division, or Twriti of Linnaeus, 

 have folds on the pillar like the VolutcB. The Trochi may 

 generally be better distinguished from the Helices by the tex- 

 ture of the shell than by the form of the mouth ; but of those 

 shells that have the mouth nearly round, it is often impossible 

 to determine whether they are Trochi or Turbines, as among 

 these many that have been called Turbo by one author are 

 called Trochus by another ; as for instance, the Turbo lineatus 

 of Turton and Donovan is the Trochus crassus of Pulteney 

 and Montagu, and many of the Turbines are neither perfectly 

 round, nor yet more nearly so than some Trochi. 



A few of the animals are described by Adanson, which differ 

 from the common Water Snails principally in being furnished 

 with three filaments on each side the foot, the use of which 

 appears to be vmknown ; they have two very slender long 

 horns, which they often put down to the ground as if to feel 

 their way ; the eyes are placed at the end of two short columns, 

 one close to the root of each horn ; most of them ha\ e a thin 

 horny operculum (see pi. 10. /. 6). 



, Linnaeus makes three divisions of this genus : first, Umbili- 

 cati erecti \ second, Impcrforati erecti ; and ihivd, Tnrriti. 



