
At our local beach we cannot find a variety of seashells, what we find are clam shells and zebra mussels. Granted, finding zebra mussels gives us a chance to tell the lad about how introducing animals to environments where they don't belong is actually quite dangerous.
I actually have a fair number of shells to show him, and count with him, with because not only did I go to Haiti on a mission trip when I was a teen (and therefore bought some and brought them home with me), I also had someone give me a whole variety of them. They've been an excellent resource for counting, sorting, tactile exploration and what not. Shells are indeed interesting.
Amanda Bennett sets up her unit studies in such a way that they are incredibly easy to use. We often copy and paste into a word processing program to cut down on paper use, but they can simply be printed off as is. They come as a ready to use PDF file. Loaded with links to various on-line resources and if you contact them about a link not working they fix it very quickly.

Day 1: Seashells--What Are They?
Day 2: Seashells and History
Day 3: Types of Seashells
Day 4: The Science of Seashells
Day 5: Seashell Party!
Day 2: Seashells and History
Day 3: Types of Seashells
Day 4: The Science of Seashells
Day 5: Seashell Party!
History, geography, science and more, all packed into one study.
(I was sent a complimentary copy in exchange for my honest opinion as part of the review team).