121 

 CHAPTER XXX. 



OF THE TROCHUS. GENUS XXVIl. 



TROCHUS. 



"Animal a Liinax. Shell univalve, spiral, somewhat conic ; aperture some- 

 what four-sided, angulated or rounded, upper side transverse compressed; 

 pillar oblique." — Linn. 



This genus is very distinct from the preceding ones, but not 

 from the two following : the animals of this and the remain- 

 ing genera have no breathing tube projecting beyond their 

 shells ; the aperture is what is called whole, i. e. the margin 

 of the lip has no hollow or canal, which the seven preceding- 

 genera have, for the breathing tube to pass through. 



It is by no means easy to discover the essential character of 

 this genus : many of the species are very nearly of a conic 

 shape, as T. niloticus, maculatus, perspectivus, &c. ; these 

 have the mouth somewhat angular, four-sided, or lozenge- 

 shaped : there are also some flat or depressed Helices, with 

 the mouth nearly of the same form ; but the Helices are com- 

 monly tliin semitransparent brittle shells, and the Trochi thick 

 opake strong shells : others, as T. Solaris, have a somewhat 

 oval mouth, acute at the ends, much like Helix Gualteriana, 

 complanata, &c. : others again vary from four-sided to half 

 round, and some almost to quite round, as T. zizyphinus, 

 Magus, Pliaraonis, Labio, Iris, muricatus. Modulus, &c. The 



R 



