BY R. M. JOHNSTON, F.L.S. 89 



not be predicted with certainty from either the shell, oper- 

 culum, external features, or anatomy of the animal." Tlicse 

 are weighty considerations. 



Mr. Petterd forgets that all systems of classification, 

 ancient and modern, ai-e more or less arbitrary and artificial, 

 whether based upon the " infallible criterion " lingual den- 

 tition, respiratory organs, muscular impressions, or external 

 form generally. Young observers, enthusiastic with a new 

 idea, are apt to forget that all fresh discoveries, however 

 valuable, only cover a small space of the whole field, and are 

 usually accompanied by fresh germs of error which must 

 also be reckoned with. Defective exo-skeleton is dead : long 

 live defective endo-skeleton ! 



So far as true progress in the exact sciences is concerned, 

 a celebrated writer has well said : " Assuredly he will not 

 be most capable of discoveries who despises the theory 

 of yesterday and swears by that of to-day ; but he who 

 sees in all theories but a means of approximating to the 

 truth and of surveying and mastering the facts for our 

 purposes." 



The best systematists of the modern school do not share 

 Mr. Petterd's distrust of our old valued friend, ihe shell and 

 its form, and some of them are even bold enough to trust to 

 its guidance in cases of conflicting evidences rather than to 

 any other singular characteristic. 



That this is the ojiinion of two of our best modern system- 

 atists (Tryon, unfortunate to science, recently deceased ; and 

 Mr. Wm. G. Biuney, who has devoted a number of years to 

 the study of the dentition and anatomy of terrestrial mol- 

 lusks), is shown by the following utterances. 



G. Tryon, who has a high opinion of lingual dentition as an 

 auxiliary aid, in his recent work on " Structural and Systematic 

 Conchology," concludes that there is "a growing conviction 

 that there are no sharply defined groups in nature; that a 

 generic character, for example, cannot be made to cover all 

 its species ; that upon its borders occur forms which partake 

 of the characters of other so-called genera, and that families, 

 orders, etc., similarly coalesce upon their confines. We may 

 anticipate a period when our larger collections, together with 

 our better knowledge of external influences and of the power 

 of adaptation to them of these creatures, shall reveal to us a 

 series of recent and fossil forms having relationship so inti- 

 mate that our present system of classification, and resulting 

 nomenclatures shall become utterly valueless. 



'•In this point of view classification is essentially arbitrary. 

 The value of a classification founded on a single organ (the 

 lingual ribbon), which does violence to other apparent uflini- 

 ties, whilst at the same time it fails of signification even in 



