GASTEROPODA. 



243 



and the whorls above the body-whorl constitute the " spire" 

 of the shell. The axis of the shell (columella) round which 

 the whorls are coiled is usually solid, when the shell is said to 

 be "imperforate;" but it is sometimes hollow, when the shell is 

 said 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. 204), 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. 205. Scalaria 

 GrcKulandica^ a Ho- 

 lostomatous Univalve. 

 Post-Pliocene. 



Fig. 206. Fusus tprnatus, a Si- 

 phonostomatous Univalve. Post- 

 Pliocene. 



In the multivalve Gasteropods, the shell is composed of 

 eight transverse imbricated plates, which succeed one another 

 from before 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. 205) 



