I used to think these were a different sort of banana slug -- and had them miscategorized accordingly -- but I saw one (not pictured) and a little while afterward, it suddenly occurred to me that the... errr... whatever you call the non-mantle portion...is obviously different.
It's said that the pneumostome is positioned differently as well, which I will now try to make a point to notice. And that the humped-up shape this one here is assuming is characteristic.
There's also a complication in that brown ones may be -- or perhaps always are -- Red Slugs instead, but I guess there isn't consensus on whether to regard Arion rufus as a species separate from Arion ater.
La persistencia de la babosa negra.
Ehh, this was better in real life. But, here. Here you go anyway. Slugs in general are a lot nicer in real life.