UNIVALVES. 57 



or horseshoe-shaped, belong to Helix; the others with the mouth 

 oval and oblique, or oval and angulated, or half-moon shaped with 

 a rib, or two -thirds of a circle with rib, belong to Planorbis. 



We can now attack the main body of the Gastropods those 

 whose shells are a raised spiral. From these we can at once pick 

 out Aporrhais, which has the pillar projecting beyond the mouth, 

 and the outer lip expanded into the winged or digitate process as 

 broad as the diameter of the shell, so clearly shown in our illustra- 

 tion of living shells. And with it we can put Cerithium and 

 Cerithiopsis, in which the pillar slightly projects, but the lip is not 

 digitate, and which can be distinguished from each other by the 

 basal groove, the first having it curved and the second having it 

 straight. This leaves us with the holostomatous shells, in which the 

 pillar does not extend beyond the mouth. Clearing off Stilifer, in 

 which the apex is divided into two, Truncatella, in which it is broken 

 off short, and Scissurella, in which there is a slit in the outer lip, we 

 can make three groups of the rest : 



Height less than breadth. 

 Height equal to breadth. 

 Height greater than breadth. 



And sub-divide the last into : 



More than double the breadth. 

 Between double and treble. 

 More than treble. 



Less than double the breadth. 



Those in which the height is less than the breadth comprise 

 certain species of Helix, in which the mouth is semi-circular; 

 Cyclostrema, in which it is nearly circular ; Vitrina, in which the 

 shell is thin and green ; Skenea, in which it is thin and opaque, with 

 a flexuous, projecting mouth ; and Zonites, which is thin, trans- 

 parent, shining, with an oblique mouth of the shape of a crescent. 

 Add to them the two flat-based genera, Adeorbis and Trochus, the 

 first of which is minute, and is further distinguishable by its waved 

 lip, and we have all this batch which are broader than they are 

 high. Briefly, they can be put as follows : 



Base flat 



Outer lip plain Trochus. 

 Outer lip waved Adeorbis. 



Base not flat- 

 Thin and green Vitrina. 

 Mouth flexuous and projecting Skenea. 

 Mouth crescentic and oblique Zonites. 

 Mouth semi-lunar Helix. 

 Mouth almost circular Cyclostrema. 



In the next group, that which are as broad as they are long, if we 



E 



