778 



DIVISIONS OF THE GASTROPODA. 



short spire ; and the columellar lip of the aperture is largely exca- 

 vated about its middle. The genus ranges from the Trias to the 

 present day. The name of Peltarion has been given to oval or 

 nearly circular calcareous plates, concave above and flattened below, 

 which are found in the Jurassic rocks, and which are now known to 

 be the opercula of species of Neritopsis. Similar opercula occur in 

 the Upper Trias and in the Tertiary rocks. The extinct genus 

 Naticopsis (fig. 674) includes thick, imperforate, naticoid shells, with 



Fig 674. — A, Naticopsis plicistria, Carboniferous Limestone. (After M'Coy.) b, Naticopsis 

 ampliata, Carboniferous Limestone. (After De Koninck.) 



a short spire and a large body-whorl. The aperture is oval, and the 

 columellar lip is callous and flattened, the outer - lip being thin. A 

 calcareous operculum is present. The genus ranges from the De- 

 vonian to the Upper Trias. 



Family 16. Helicinid^. — This family includes the recent Heli- 

 cina and its allies, in which the shell is helicoid and globular, with a 

 short spire, the interior turns of the spire being absorbed in course 

 of growth. The aperture is semicircular, and a horny or calcareous 

 operculum is present. This family has been generally included 

 among the Pulmogastropoda, upon the ground that the breathing- 

 organ is in the form of a pulmonary chamber, and the animal is 

 terrestrial ; but the true relationships of the group appear to be with 

 the NeritidcB. All the undoubted forms of this family are Recent ; 

 but Fischer is inclined to place here the Carboniferous genus 

 Dawsonella, which has been commonly included among the Heli- 

 cidcz. In this genus the shell is like that of Helix in shape, with a 

 small aperture, the outer lip of which is thickened, while the col- 

 umellar lip is expanded into a large callosity, which covers the 

 whole umbilical region. 



Family 17. Naticid^:. — In this family the shell is globular, of 

 few whorls, with a small spire (fig. 675) ; the aperture oval, rounded 

 in front, and narrow behind, with an acute outer lip, and commonly 

 with a callous inner lip. The foot is very large, and the mantle- 

 lobes hide more or less of the shell. The operculum is pauci- 



