What makes a really good marrow bone? Every now and again I really crave some crustinis with marrow.  I've had mixed success trying to find the ones where the marrow can easily come out.  There's usually sinewy ossificans and it makes it hard to get all the marrow out easily.

What are some reasons that some bones don't have ossified material inside and some that do?  Does it happen as the animals get older?
Some butchers will custom cut beef leg bones. You can specify a short cut for the marrow bone so that your retrieval device can fully reach all of your marrow. I work at a charcuterie and we frequently cut marrow bones to order.