I was absolutely convinced DS was going to start crawling this week. He's been getting up onto all fours and sort of rocking (you know, where they look like they are humping something). But he's suddenly worked out that if he gets into this position and walks his hands backwards he can sit himself up instead. Now if he wants something he just flops onto his tummy, sits up, flops again until he gets there. He's also just started trying to pull himself up - usually using me when I'm sitting on the floor. I'd be interested to hear some predictions from people who've actually done this before!!
NB I'm not a competitive parent, this is just a bit of fun!!
My DS skipped proper crawling, just dragged himself round by his arems comando style for a couple of months, then walked. I think he actually was walking before he learnt to get himself in the sitting position.
My Ds just rolled everywhere then at 12/13 months he rolled and pulled himself up to his feet. Everyone told me "he's just going to walk, he will never crawl". 13.5 months he started to crawl, he thought, oh this will do me and didn't bother to walk until 17 months!
My DS1 did the rocking thing for weeks! I began to despair that he would ever crawl or indeed move any great distance.
He used to have his ankles crossed though and I don't think he could work out how to get them uncrossed.
My bet would be that if your DS has worked out how to get onto all fours he will crawl at some point.
I was a bottom shuffler and apparently I never managed the one all fours thing (although I would like to point out I can do it now, just in case you are wondering so I had to have learnt it at some point.)