The comparison scores every group at once. This page is for the moment you already have a pair of names in mind — mayfly or stonefly, scud or sowbug, shad or alewife — and want the structures that keep them apart, in one look. Nothing here is a current hatch.
The differences, and nothing else
Best separating check: ShellAt a glance It separates Freshwater snail from Clam (small freshwater bivalve) cleanly, and every pair of groups still in play differs on it and you can check it at a glance.
What to look at
Freshwater snail
Clam (small freshwater bivalve)
ShelldecisiveAt a glance
A single coiled shell with a soft foot
Two hinged shell halves that close
Crawling on rock or debrisdecisiveWhere it was
Yes
No
Reported size
2–70 mm
2–20 mm
How it moves
glides slowly on a muscular foot
ploughs slowly through sediment on a muscular foot; pulls its two shells shut with strong muscles
The same for both: water, life stages, seasonality.
Rows come from the reviewed diagnostic weights the comparison scores with. “Decisive” means one answer puts them at least four points apart — the carrier's own rule for a structure that separates a pair.
The two entries, side by side
Gastropoda
Freshwater snail
One shell, usually coiled, and a foot out of it.
First check
Is it a single shell — coiled, or a low cone in a limpet — with a foot rather than a head with legs coming out of it?
What it is
A single shell with a soft foot gliding out of it is one of the few things in this guide that is hard to confuse once you have it in your hand. The shell is usually coiled; a freshwater limpet's is a low cone.
Stages
Every stage — juveniles are the same animal, smaller (2–70 mm (most under 20 mm))
Water
spring, stream, river, lake-margin, lake-open, reservoir
Separates from Clam (small freshwater bivalve)
One shell or two. A snail's shell is a single coil; a clam is two hinged halves that close on the animal.
Typical windows
No family-level window in this layer.
Patterns (correspondence)
None mapped yet.
Limits
'Snail' and no finer. This guide does not attempt the gilled/lunged split, because the character that decides it, the operculum, is hard to see on a small shell.
Two halves and a hinge. That is as far as this goes.
First check
Two halves with a hinge along the top edge, or one coiled shell?
What it is
Two hinged shell halves that close on the animal. In the hand it is quick: you can see the seam and feel the hinge, and a live one pulls its shell shut. The one two-shelled lookalike, the clam shrimp, has jointed legs and feathery antennae.
Stages
Every stage — young are miniature adults (2–20 mm for fingernail and pea clams, most under 13 mm)
Water
spring, stream, river, lake-margin, lake-open, reservoir
Separates from Freshwater snail
A snail is one coiled shell; a clam is two hinged halves.
Typical windows
No family-level window in this layer.
Patterns (correspondence)
None mapped yet.
Limits
Honest at 'small bivalve' and no finer. A young Asian clam can be mistaken for a fingernail clam; raised ridges and a thick shell point to the Asian clam, but this guide does not make that call.