Thanks for the link. They are Brachycybe producta.

since these are our first, can you give an explanation of how you determined species? Thanks.

Assuming the website that is linked to from BugGuide (this one) is a complete species list, I determined that there are only two possibilities - B. producta and B. potterinus. Two other species are only listed as occuring in Southern California.

B. producta is listed as specifically occuring in Marin County on that page, and when I did a quick Google search on the species, I found this picture, which look exactly like the millipedes I found (the color in that photo is a bit off). Of course, since I cannot find any pictures of B. potterinus to compare, I cannot rule out that species for certain, but at this point in time it is looking like B. producta is in all likelihood the species I found here.

If, however, the species list above is incomplete or incorrect, I could be way off and it might be neither of those species. It would be great if anyone who has experience with Californian millipedes weigh in with some additional information/advice.

Thanks for the explanation. You can either move it to genus page or wait for me to make a species page for you, and then you can move it there.

I'm still pretty new to the website and I couldn't figure out how to move these images, so I uploaded new ones to the guide. You can move/delete whichever ones you want to, though.