Hi David,
 
Maybe it was something it ate? I know that sounds like a flip remark,
but since all mollusc manufacture their shell material, perhaps the S.
ranius you collected ingested something that caused it to secrete black
pigment into its shell building materials. Come on you science guys, I
know you are laughing, but what do you think?
 
(I collected a lovely C. caputserpentis several years ago that had
included green pigment in its dorsal area)
 
Patti in New Mexico
 
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