xii PREFACE TO THE SECOND EDITION. 



few instances, have the opportunity of examining 

 indeed many of the shells composing his 

 genera are very rare, and they could only know 

 them by pictures ; the generic distinctions also 

 are often so minute that they would elude the 

 observations of minds unpractised in scientific 

 details. There is on the contrary something in 

 the simplicity and clearness with which the great 

 Swede seized and defined the broad outlines of 

 nature peculiarly suited to the capacities of the 

 young; and they may be led with very little 

 assistance, to group the shells according to his 

 arrangement, and to draw out from their own 

 observation the generic characters. Nor will an 

 acquaintance with the Linnsean system be with- 

 out its value, when they may desire to study a 

 more perfect classification than his; they will 

 find the task of subdividing, separating, and 

 arranging very much more easy than if they had 

 to work upon materials altogether new to them ; 

 they will have points established in their minds 

 to which they may compare and attach the new 

 ideas presented ; their perceptions will be quick- 

 ened to discover similarities and differences ; and 

 they will find many of the difficulties that em- 

 barrass the student at the very threshold of 

 science, overcome by their previous lessons. 

 Some again have objected to the subject of the 

 lessons, as being so much less interesting than 



