12 VIVIPARA. 
since been used as genera by Mr. W. G. Binney 
in his recent monograph published by the 
Smithsonian Institution. I prefer to continue 
to regard these groups as subgenera, because 
some of the characters of the soft parts de- 
scribed by Mr. Binney are of very doubtful 
distinctive value in my estimation. I may 
particularly adduce the descriptions of the 
animals of Vivipara and Melantho. 
“Vivipara. Foot of moderate size, thick, 
not produced beyond the snout. Colors very 
dark.” Fig. 23, Binney’s Viviparide. 
“Melantho. Foot large, rather thin, broad, 
much produced beyond the snout. Colors 
rather light,” ete. Fig. 68, same work. 
Now either description will apply to either 
genus. When the animal is at rest description 
No. 1 will apply, but when it walks it becomes 
No. 2. Thus Say’s decisa belongs to the genus 
Vivipara when quiescent, but has the remark- 
able faculty of walking and reaching into the 
genus Melantho ! 
Those who are curious about this matter, 
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