Till 



hitherto submitted. It therefore seems proper, especially in view 

 of the fact that this article will have a circulation among many 

 persons who are interested in the collection and study of shells, 

 but who have never paid especial attention to the principles of 

 classification involved in the arrangement of the niollusks, to 

 offer a few prefatory remarks on Taxonomy, or the science of 

 classification, especially so far as those animals are concerned, 

 and to answer the questions that may arise as to why some 

 combinations are made. 



PRIMARY DIVISIONS. 



The classes of Mollusks are by no means allied to each other 

 in equal degree ; there are two series that differ very widely, and 

 which have been regarded by many of the best naturalists as 

 primary groups of the animal kingdom ; that is, sub-kingdoms or 

 branches. The great majority of the representatives of each 

 of such groups do indeed offer so many special characteristics, 

 and so widely differ from those of the other series, that perhaps 

 the arguments in favor of such a view may be more weighty 

 than those for the opposite. But the members of one class 

 (Tunicata) seem to be in some respects intermediate or at least 

 to narrow the chasm that would otherwise exist between the 

 two, although their affinities are not regarded as dubious by most. 



It has been found, after due investigation, that the central 

 nervous system offers in its modifications in the Mollusks, as in 

 the Vertebrates, the best criteria of relationship, and on the num- 

 ber of ganglia have been based the division thereof into the two 

 primary groups, MOLLUSCA VERA and MOLLUSCOIDEA ; in the 

 former (Mollusca vera), there are three well developed pairs of 

 ganglia the cerebral, the pedal, and the so-called branchial (or 

 parieto-splanchnic of Huxley) each pair being united by com- 

 missures ; in the latter (Molluscoidea), there is but one well de- 

 veloped pair, homologous with the pedal ganglia of the true Mol- 

 lusks. Prof. Huxley, that very able biologist who has so 

 much contributed by his clear mind and convincing logic to the 

 education of the younger naturalists of the present day, has well 

 remarked on the impossibility, or at least difficulty not yet sur- 

 mounted, of the enunciation of a diagnosis which will combine 

 the two divisions, and distinguish that combination from others. 



