MANUAL OF NATURAL HISTOEY 129 



fissure into two creeping disks ; operculum 

 none; shell ear-shaped, spire minute; aperture 

 large. 



15. FAMILY. Coriocellas (Lamellariidse). Eyes at 



bases of tentacles ; mantle very large, en- 

 tirely covering the shell, notched in front ; 

 operculum none ; shell thin, semi-pellucid, 

 ear-shaped ; aperture wide. 



16. FAMILY. Sea-Snails (Naticidse). Eyes none; 



mantle enclosed ; foot very large, produced 

 in front, operculigerous, lobe greatly deve- 

 loped, partly covering the shell ; operculum 

 sub-spiral ; shell smooth, spiral ; aperture 

 semi-lunar, entire in front. 



17. FAMILY. Helmet-Shells (Cassididse). Teeth nu- 



merous, similar, in many rows ; mantle en- 

 closed, with a recurved siphon ; operculum 

 annular ; shell ventricose, whorls often variced; 

 aperture with a recurved canal, or a notch in 

 front ; outer lip thickened. 



18. FAMILY. Wentletraps (Scalidse). Mantle en- 



closed, without a siphon ; foot moderate ; 

 eyes on outer side of tentacles ; operculum 

 horny, spiral ; shell turreted, variced ; aper- 

 ture round, without any canal. 



19. FAMILY. Awl-Shells (Terebridse). Mantle en- 



closed, with an elongated siphon ; eyes on tip 

 of tentacles or wanting ; tentacles small ; foot 

 small ; operculum annular, nucleus apical ; 

 shell turreted ; outer lip thin, not variced. 

 (Terebrina, Morch ; Acusidce, Gray.) 



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