PROSOBRANCHIATA. 



777 



In the genus Nerita (fig. 673) the shell is thick, with a broad 

 columella, the inner edge of which is straight and toothed. The 

 outer lip is thickened and often denticulated internally. The true 

 Nerites are inhabitants of warm seas ; and typical forms of the genus 



Fig. 672. — Xenophora {Phorzis) canaliculata. Cretaceous. 



appear first in the Cretaceous rocks, though nearly related types are 

 known from the Triassic and Jurassic rocks. 



The Triassic and Jurassic genus Neritodomus closely resembles Nerita, 

 but the columellar lip of the aperture is thickened and callous, and is 

 without teeth. In the nearly allied Merit oma, of the Jurassic rocks, the 

 shell is thick and ventricose, and the columellar lip is callous, and is not 

 toothed, while there is a notch or sinus in the middle of the outer lip. 



Fig. 673. — Nerita (Velates) Schmideliana. Eocene Tertiary. 



The genus Meriting again, includes the so-called " Fresh-water Nerites," 

 which agree in most characters with Merita, but inhabit fresh or brackish 

 waters, and have a comparatively thin smooth shell. The oldest forms 

 of Meritina seem to appear in the Lias, and the genus is abundantly 

 represented in the fresh-water deposits of the Tertiary period, while 

 many recent forms exist. Lastly, the genus Pileohcs comprises small 

 limpet-shaped shells, with a semilunar aperture. The range of the 

 genus is from the Jurassic to the Eocene. 



Family 15. Neritopsflve. — In this family the shell resembles 

 that of Nerita or Natica in shape, and is thick and imperforate. 

 The aperture is entire, semilunate, or oval in form ; and there is a 

 thick shelly operculum, oval in shape, and non-spiral. In Neritopsis 

 the shell is shaped like that of Nerita, of few volutions, and with a 



