<<"frog shells", wentletraps, and "Tun shells" which make
no sense at all in modern-day English>>
 
Frog shells are named for the squat, heavy, angular appearance of certain
species, such as Bursa tuberosissima, Bursa lamarckii, and especially Bursa
bufonia which, in my opinion, is the most "frog-like" in shape of any species.
The name "bufonia" incidentally means "toad-like".  Other species names in the
family also refer to frogs and toads (Bufo = toad; rana = frog).