418 THE MEDALS OF CREATION. Cap. XI. 
some species it is composed of several pieces. The most 
simple form of shell is that of the hollow cone, of which the 
Patella, or limpet, affords an example ; and in the more 
complicated modifications, the cone is twisted, or convoluted 
spirally, either in the same plane as in the Planorbis of our 
rivers, or obliquely, as in by far the greater number of 
Eien. 133. FossIb SHELLS OF GASTEROPODA. 
Fig. 1.—PALUDINA FLUVIORUM. Wealden. 
2.—LIMN#A LonGiscaTa. Tertiary. Isle of Wight. 
3.—CERITHIUM LAPIDORUM. Tertiary. Grignon. 
4.—Fusus CONTRARIUS. Crag. Essex. 
species. The direction of the spire is generally from left 
to right, the aperture being dextral to the observer when 
the shell is placed with its apex uppermost, asin Lign. 133, 
jigs. 1, 2, 3; but in a few species the spire turns in the 
opposite manner, and the mouth or aperture is to the left, 
or sinistral, as in Lign. 133, jig. 4. In consequence of the ~ 
form of the aperture of the shell, the entire or notched 
