4 GASTEROPODA. 



to be " perforated," and the aperture of the axis near the 

 mouth of the shell is called the " umbilicus." The margin of 

 the " aperture " of the shell is termed the " peristome," and 

 is composed of an outer and inner lip (fig. 383), of which 

 the former is often expanded or fringed with spines. When 

 these expansions or fringes are periodically formed, the place 

 of the mouth of the shell at different stages of its growth is 

 marked by ridges or rows of spines, which cross the whorls, 

 and are called "varices." The animal withdraws into its 

 shell by a retractor muscle, which passes into the foot, or is 

 attached to the operculum ; its scar or impression being 

 placed, in the spiral Univalves, upon the columella. 



Fig. 384. — Scalaria 

 Grcenlandica, a Ho- 

 lostomatous Univalve. 

 Post-Pliooene. 



Fig. 385. — Fitsus tornatiLS, a Si- 

 plioiiostomatous Univalve. Post- 

 Pliocene. 



In the multivalve Gasteropods the shell is composed of 

 eight transverse imbricated plates, which succeed one another 

 from Ijefore backwards, and are embedded in the leathery or 

 fibrous border of the mantle, which may be plain, or may be 

 beset with bristles, spines, or scales. 



In the marine Univalves two important variations exist in 

 the form of the mouth of the shell. In one group (fig. 384) 

 the mouth of the shell is unbroken or " entire," not having 



