6 SYSTEMATIC DISTRIBUTION. 



In describing shells, they should be regarded in 

 their natural position ; that is to say, in the way in 

 which they are placed on the animal. Thus the part 

 of the shell over the head of the animal is called the 

 front, and that over the tail the back, of the shell ; 

 and the left and right sides of the shell correspond 

 with the left and right sides of the animal. 



This is exceedingly easy to be determined in the 

 univalve shells, because the apex of the shelly cone, 

 whether it be simply conical or spiral, in all univalves 

 (except Patella and Lottia) is over the hinder part of 

 the animal; therefore, if a shell is placed on its mouth, 

 with the apex towards the spectator, the parts of the 

 shell will correspond with the position of the person 

 who is looking at it. 



As all shells are formed of a shelly cone (which, 

 when very long, is generally rolled round an imagi- 

 nary axis, for the purpose of diminishing the space 

 that it occupies ; but when it is short, is sometimes 

 only slightly recurved, as in the Ancylus and Vel- 

 letia), in order to maintain a similarity of terms for 

 the same thing in these two forms, all the lines or 

 grooves which pass from the apex of the cone to the 

 mouth, and which are caused by some permanent 

 modifications of the edge of the mantle, are called 

 longitudinal or spiral, and all the lines which are 

 paralled to the edge of the mouth of the shell, and 

 which, in fact, are generally marks of its growth, or 

 are caused by some periodical development of the 

 margin of the mantle, are designated as concentric 

 or transverse. Thus the striae on the Cyclostoma 

 elegans and Planorbis albus are longitudinal or spiral, 



