In my research, nothing said mountain biking was impermissible…however, I found out that, past the saddle with the Espinosa Trail and the road to Mount Pinos, to describe the remaining portion of the trail to Corte Madera as “not ideal for mountain biking” to be true.  Thick brush, not very wide trail, some steep moments…oh I still hiked my bike up and I rode down as much as I could- was still great, though my arms and legs are all cut up and itchy because of my own lack of preparedness- I was planning on a different hike-n-bike, one I knew had much wider trails (Santiago Peak via Maple Springs), but then decided do not drive twice as long for an even longer hike/ride that day.  Vivi e impari!