THE GASTROPODA 95 



it is therefore described as univalve. The simplest 

 form of this shell is an elliptical cone, the aperture occu- 

 pying the whole base ; but in the great majority of cases 

 the shell is a cone coiled in a helicoid spiral. 



In many cases there is a horny or calcareous plate 

 (operculum) which closes the apertures of the shell when 

 the animal withdraws into it. This, however, is never 

 hinged to the shell, and the idea that it represents a 

 second valve like that of lamellibranchs has no justifica- 

 tion whatever. 



Gastropods are the only class of molluscs that inhabit 

 the dry land, as well as fresh and salt waters. They are 

 known from all the geological systems,' but it was only in 

 the Cainozoic era that they became really abundant. 

 From the Eocene period down to the present day, they 

 constitute, with lamellibranchs, the main part of the 

 invertebrate fauna preserved as fossils. 



i. Emarginula fissura (Fig. 26), found fossil in the 

 crags of East Anglia and still living in British seas, is an 

 example of an almost symmetrical gastropod. It is shaped 

 like a cone, with an elliptical base and the apex bent over 

 in the plane of the major axis, which thus appears to be 

 a plane of symmetry. More careful examination with 

 a lens shows, especially in youthful examples, that the 

 apex is curled in a spiral to one side, so that there is no 

 perfect plane of symmetry. The side on which the 

 spiral lies is the right side, the direction in which the 

 apex points being posterior {with reference to the animal's 

 anatomy). The anterior margin has a deep, parallel- 

 sided slit. The outer surface is ornamented by alter- 



