The frog shell is not extremely pretty as it tends to be covered in “limy deposits”.  The St. Thomas Frog Shell (Bursa thomae) got it’s name because it was first discovered in St. Thomas in the Virgin Islands. It’s a small shell, growing only to a little over an inch long.

Another frog shell – the California Frog Shell (Bursa californica) is commonly found in Baja California where it is frequently pulled up in crab and lobster traps. This species is larger and can be up to 5 inches in length.
Info taken from National Audubon Society’s Field Guide to Shells

See another photo of a frog shell – The Yellow Mouthed – here.