Slugs and Snails in Colorado

These are small fresh water snails, of the family Physidae. They are air breathers, and live in shallow ponds. These snails are different in that they have shells that are sinistral, which means that if you hold the shell such that the spire is pointing up, then the aperture is on the left-hand side.
The identification was made by Peter G. Walker, of the Colorado Division of Wildlife. He corrected my earlier incorrect identification.
They were found in a pond at the Fountain Creek Nature Center on 24 August 2008. The two larger ones are about 5mm in length.
Kingdom: Animalia
Phylum: Mollusca
Class: Gastropoda
Subclass: Orthogastropoda
Superorder: Heterobranchia
Order: Pulmonata
Superfamily: Planorboidea
Family: Physidae
Genus: Physa

A Grey Garden Slug, Some sources say this is Agriolimax reticulatus, others say it is Deroceras reticulatum. This one is about 1" long when stetched out. It has a breathing hole on it's right side.
Kingdom: Animalia
Phylum: Mollusca
Class: Gastropoda
Order: Pulmonata
Family: Agriolimacidae
Genus: Deroceras
Species: D. reticulatum