In Marin County it could either be a Southern alligator lizard (California subspecies) or a Northern (I believe San Francisco subspecies in Marin)

The way to tell them apart is by the lines on the belly.

If they run down the middle of the scales, it is southern - between the scales, northern.

Here are photos of San Francisco subspecies of Northern:

www.californiaherps.com/lizards/pages/e.c.coerulea.html

Here are photos of California subspecies of Southern:

www.californiaherps.com/lizards/pages/e.m.multicarinata.html
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6.8 L. getula californiae - 11 eggs (Cal. King)
1.2 L. getula nigrita (MBK)
1.0 L. getula floridana (Brooksi)
1.0 Pantherophis guttatus guttatus (Corn)
0.1 Pituophis catenifer catenifer (Pacific gopher)
3.3 Elgaria multicarinata multicarinata - 14 eggs (Cal. Alligator Lizard)