This week's Sunday Spider is a ground spider of the family Gnaphosidae. There are hundreds of species in this family and most are nearly impossible to identify without examining their genitalia under a microscope.
Ground spiders are found most often under rocks or logs, where they build silken retreats and emerge only to hunt. Some species wander indoors when the weather turns cold. Ground spiders are not considered to be harmful to humans or pets.
I found this little guy inside my snail tank a couple of months ago. It deftly maneuvered in and out of the ventilation holes in the tank lid as it was exploring. I was able to get this quick pic when it briefly came out onto the outside of the lid. I have no idea how it got into the tank in the first place.