I've been reading the Keipl & Alf book "Recent Xenophorae" again and
struggling to make head or tail of it. Do I gather that X. robusta is the
same species as X. conchyliophora, just a different type? Then I'm wondering
what the heck is a type, as opposed to a subspecies?

I want to know more about carrier shells, but much of this book is Greek to
me. I'm no dim bulb, but I don't have the science background. Clearly I need
a better understanding of taxonomy or claudistics or whatever. Can anyone
reccomend a good book on the subject to get me started?

Thanks,
Ellen