170 THE AMERICAN MIDLAND NATURALIST 



observations do not agree with calculations from Walker's illustra- 

 .tion of this species which shows the female to be relatively higher. 

 They do agree in assigning the female a greater posterior length 

 of shell. 



Eurynia: "Male shell drawn out behind and ends in a blunt 

 point about midway up from the base. Female shell has long 

 rounded marsupial swelling, ending in a blunt point ^3 way up from 

 the base." This data agrees with m)'' measurements, as it maybe 

 inferred the male is longer, the marsupial swelling may be in 

 evidence from the inflation of the shell at the point measured. 



L. luteola: ' ' Female shell with most decided marsupial swelling ; 

 here blunt posterior point is somewhat higher up, (f), of height 

 than that of male, (about halfway), and is usually more inflated." 

 My results show the female as a higher degree of inflation, and is 

 besides relatively longer. 



L. ovata: "Female shell slightly inflated post-basal region," 

 etc. As the recorded dimorphism for this genus is practically the 

 same as for these last 2 species, I need only point out the full accord 

 with my results. 



The value of quantitative studies of the morphological charac- 

 teristics of shells is best appreciated when cases are brought back 

 to mind where new genera and species had to be founded on the 

 anatomy of soft parts alone so great was the superficial resem- 

 blance in some case's between what turn out to be entirely different 

 animals. If as a general proposition, it be admitted that the system- 

 atist should be able to find in any organism specific characters 

 distributed from the most minute anatomy to the coarsest features 

 of morphology, any such convergent phenomena as described above 

 could be eleminated as each species of shell could be expected to 

 vary in morphological characters around its own mean. It is to 

 be hoped that the practice of publishing the more usual dimensions 

 for both sexes of shells will be continued, that the ultimate philo- 

 sophic trend of all Biological Science may have ample data for the 

 consideration of the never-dying and always-puzzling question 

 of the environment. 



(c.) Source of Error. 



Paucity of material compelled me to use in some species a few 

 shells from Lake Erie, (most were from the Upper Ohio Drainage), 

 although it is in some of the measured shell characteristics, as I 

 expect to show, that the Lake Erie shells differ from those of the 



