Here are some of the non-native species I've encounted:
|Photo by Aiwok: http://commons.wikimedia.org/wiki/File:Discus_rotundatus_2.JPG|
Limax Maximus : Leopard Slug
|Photo by Jonathan Feinberg http://commons.wikimedia.org/wiki/File:Leopard_Slug_Somerville_MA.jpeg|
Trochulus hispidus : Hairy Snail
|Photo by James K. Lindsey http://commons.wikimedia.org/wiki/File:Trichia.hispida.jpg|
Carychium minimum : Herald Thorn
|Photo by snailmail http://commons.wikimedia.org/wiki/File:Carichium_minimum.jpg|
I only found empties of this snail in stream drift from Buttermilk Falls. However, I read a paper about the snail being naturalized in wet areas around Beebe Lake on the Cornell campus. This is a tiny snail-- somewhere between 1.6mm and 2.2mm.
Other species about:
Cepaea nemoralis- around the Lab of Ornithology. Rose and yellow variants, as well as spiral lined. There may also be C. hortensis mixed in. That or it's some C. nemoralis juveniles reaching to about C. hortensis size and not yet developing the brown lip. I'll have to check harder next year. I tend to gloss over non-native species, not giving them the attention I do natives.
Oxychilus allarius- possibly. Cayuga Heights and near Lab of Ornithology
Arion species- Definitely subfuscus and possibly distinctus. Never pay much attention to ugly Arion slugs.
Deroceras reticulum- another ugly alien slug, everywhere.