90 CRITICAL OBSERVATIONS OX RECENT CONTRIBUTIONS, ETC. 



one of the most important functions with which it is con- 

 nected, in that it does not enable us to certainly separate the 

 phjtophugous from the zoopbagos animals, may be seriously 

 questioned. 



" We have many most important characters of the mollusks 

 which impress themselves upon their shells, so that they are 

 in accord, and enable us to predicate reciprocally their 

 relationships; and such characters appear to be much more 

 useful for classification. ' Biuney exjiresses himself in a 

 similar way, and states briefly : "If it be proposed that a 

 single arbitrary standard shall be used because it is arbitrary 

 . . . . then the standard selected should be the most 

 universal and the most apparent, namely, the shell." 



Binncy, who has devoted many years to the si)ccial study of 

 dentition, goes so far as to say, " Is it not impertinent to 

 make use of a few hundred observations of an organ which 

 only pervades a portion of the moUusca, to establish a classi- 

 fication which is frequently in violent contrast with natural 

 aflBnities ascertained by long examination of all the species, 

 recent and fossil ? " 



Enough has been stated to show that we have no new 

 " divining rod " to help us in classification difficulties. Wide 

 careful comparison of all characters are certainly necessary, 

 but so long as local workers only trouble themselves to single 

 out extremes of each type for the information of others, so 

 long will a satisfactory classification of our shells be a thing 

 of the future. 



Local workers would better advance the cause of science if 

 more regard were paid to the study of the variability of char- 

 acters of the shell and of the animal. Little is known yet how 

 far the denticulae of the lingual ribbon varies in animals of 

 the same genus, and this must be well studied in every group 

 before we can depend upon their form and numbers for 

 determining the limits of a genus. 



Is our knowledge of the constancy of form and number of 

 denticles on the median tooth of fresh water shells wide enough 

 to enable us to rely upon its indications alone for marking the 

 limits of a genus? This is a most pertinent question. Some 

 of our best classifiers, who have tested this matter system- 

 atically, insist that reliance upon such characters are decep- 

 tive, and are not so reliable as the more obvious ones. 



