If that's really what you're feeling, your date is probably anorexic.  You are more likely being milked by her vaginal muscles.  Ask her if she does Kegels.

Well....

The only bone in the area you might be feeling is the pubic bone located on the front of the vagina just past the vaginal entrance.  

I suspect what you are feeling is the vaginal musculature which can feel somewhat ribbed. Deeper you may be able to feel the cervical os, which is a dome-like structure projecting from the uterine canal into the top of the vagina. Also, around the cervical os is a frisbee-shaped area of muscle called the fornix.

Sorry, no bones about it.

Some women have more prominent rugae.

Another possibility

Something I learned  during a recent drama-fest on the Boston boar

So pay attention and take notes.

 
They will be a test afterwards

You need some serious sex education.

Now may I suggest you go to the library and pick up some anatomy and sexual education books. Or as an alternative there are plenty of information available online. Your lovers, and this board will thank you for it. Vaginal bones indeed.

Hope link works

It was a lil difficult getting in. She told me she  did a lot of kegals. I told her to try an relax so I could get in. She did and so did I.