20 



INTRODUCTION. 



the shell is to be measured for length, a indicates the ante- 

 rior, p the posterior. The line d (fig. 23), from the apex to 

 the base, is the depth. The line b (fig. 28), is in the direc- 

 tion of the breadth. 



Irregular non-symmetrical Univalves. 



Serpuliform shells are irregularly twisted {tortuous) hollow 

 tubes, which were formerly considered to have been secreted 

 by a kind of worm, but now known to be the shells of true 

 Molluscs, of a kind not very widely differing from those which 

 have regularly spiral shells. The greater part of these are 

 attached to foreign bodies, or to each other in groups. Some 

 are attached by the whole length of the shell, they are then 

 said to be decumbent. Some of these are coiled round like 

 the Spirorbis, the little white shell seen on the carapace of the 

 Lobster or on leaves of sea-weeds ; they are then said to be 

 discoidal ; others again, such as the Vermetus, approach more 

 nearly to the spiral form. The deviation from the regular 

 spire only taking place after the few first volutions. 



SPIRAL NON-SYMMETRICAL UNIVALVES. 



As these constitute the largest 

 class, it will be necessary to dwell 

 upon them in detail. First as to 

 measurement. 



The length is measured from the 

 apex, to that part of the aperture a 

 / (fig. 24), at the greatest distance 

 from it. The breadth is in the oppo- 

 site direction. The anterior, or 

 front part of the aperture, is marked 

 «, where the head of the animal pro- 

 trudes. 



