INTRODUCTION. 5 



On account of these and other considerations, it has been 

 thought advisable that the present undertaking should bear a 

 purely conchological character. The peculiarities of the shells 

 alone being detailed for the assistance of those who collect and 

 study them, while at the same time, in deciding upon their 

 affinities and places, in the arrangement, it will be necessary to 

 take advantage of the conclusion to which those have arrived, 

 who have studied the animal in all its parts. And the con- 

 viction must be expressed, that if ever a complete Natural 

 System shall be formed it will result from the labours of the 

 last mentioned class of naturalists. 



DEFINITION OF A SHELL. 



Before entering minutely into the description of shells, it 

 will be necessary to distinguish from the true testaceous 

 Mollusca two kinds of animals which have formerly been 

 associated with them. Of these, the first is the class of 

 Crustacea, consisting of crabs, crayfish, &c. These differ 

 from shell-fish, not only in structure and chemical composition, 

 but also in the fact that the animal has jointed limbs, and that 

 the substance of the flesh is inseparable from the hard ex- 

 ternal covering, which invests each particular joint as with a 

 sheath ; whereas the Molluscous animal is but partially at- 

 tached to its shell, from which it possesses the power of partly 

 withdrawing and returning. The second class is that to which 

 the sea-urchin, or Echinus, belongs, of which there are many 

 genera and species. The testaceous covering of Echini is 

 composed of a number of small pieces, placed edge to edge, 

 forming a more or less globular external covering to the flesh, 

 which is supported in the centre by a number of bones leaning 

 upon each other in a pyramidal form. The test is of a fibrous 

 texture, guarded on the outside with moveable spines, which 

 turn on ball and socket joints. 



