Here are shots of my 99% correct mold. I'll detail what Ii would change after the pics:
Quick explanation of what you are seeing:
anything blue is silicone
anything silicone was once clay
white stuff is urethane plastic with chopped fibers call plasti-paste which creates a rigid shell to keep the soft mold from deforming when filled with expanding foam.

The holes you see here are where i poured silicone into the mold, they have no other function. The big one is where i poured it in, and the smaller ones are vents for the release of air, so it doesn't get trapped and make a void in the mold.

Here you can the front support shell has two parts, but inside it is one piece of silicone that fits in here.

Here is the view from behind. The dividing line between the mold halves is behind the teeth line, so there is no seam visable on the front. You can see keys here on the silicone edge where it locks into the mold. Good for most molds, but bad for this becaseu of its odd U shape. Don't key the silicone to the shell, just to the the other silicone half.

Here you can the original still in the mold half. The back support is one piece.

This is where the casting material is poured. It was made with a glue stick that was attached to the jaw during the mold making, then removed after the silicone cured. It can easily be plugged after it is filled with casting material with another glue stick. Those holes you see in the shell were vents from the silicone pouring to let air escape, however I have drilled thru them and thru the silicone. This creates vents for the the foam to expand and escape.

Here you can see the spout and ow it fits thru the support shell. If you look at the shell you'll see the are criss-crossing lines which are the mold keys. If i were to remake this mold, I would not have those keys. TO remove the jaw after casting without breaking it, you need to slide it out of the support. I cut off the keys on the silicone part and can pull it straight forward. I would just key the silicone halves to each other. You can see two small round holes behind the edge of the jaw in the silicone. Just make more of these to keep the halves aligned.
Check out his matrix mold how-to, it really illustrates the ideas used in this mold:
Hellboy bust and matrix mold making tutorial - Page 2 - ConceptArt.org Forums