CHAPTER XXXI: THE SCREW SHELLS 

 TOWER SHELLS 



Family Turritellid/€ 



Shell a long slender spire of many whorls with revolving 

 striae and fine, curved lines of growth ; mouth oval, or four-angled; 

 lip thin; operculum spiral, horny; head with broad snout; eyes 

 on bases of long spreading tentacles; mantle edge fringed; gill 

 plume long, single; foot short, truncate in front, narrowed behind, 

 grooved underneath, A marine family. 



Genus TURRITELLA, Lam. 



Characters of the family. Four hundred fossil and one 

 hundred living species, chiefly in the Old World. Very few on 

 American beaches. A peculiarity of this genus is that the upper 

 fourth of the shell is always empty and divided by a septum 

 at each half-turn. 



The Great Screw Shell {T. terehra, Linn.) has a most 

 elegantly turned spire, tapering to a needle point, its sixteen 

 whorls strongly grooved and ridged as if done in a lathe. The 

 pale surface is stained with orange, or clouded all over with 

 fulvous brown. The largest ones I have seen are five inches 

 long, with a breadth at base of more than one inch. The same 

 proportions hold in the smaller specimens. They come from the 

 Philippine Islands. 



The idea of the screw was suggested to the philosopher 

 Archimedes by the spiral shell of Turritella terehra. 



The Marbled Tower Shell (T. marmoraius, Keiner) is an 

 obelisk of many flattened whorls, finely marked with growth 

 stride. It tapers but little, and the apex is blunt. The surface 

 is clouded and reticulated with lurid chocolate shades. Length, 

 6 to 8 inches. 



Habitat. — Philippines. 



The Girdled Screw Shell (7. cingulata, Sby.) is a slim 



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