First let me say hello, this is my first post here. 
I have been trying to follow the guidelines in this tutorial. It describes a method of modelling a head after front and side pictures. It mirrors half the head, so that you only have to work on one side.
I figured I’d better start from scratch, so I found a picture of a skull, and I’ve been working on it for a few days now. The problem is that without using subsurface divisions, it looks pretty bad. But if I turn them on, I get a very ugly seam between the two halves.
In this picture you can see the skull with no subsurf (on top), 1 subdivision (bottom left) and 2 subdivisions (bottom right):
-> Larger image
I’ve tried joining them to one object (ctrl-j), and removing the duplicates to connect the meshes, but I get the same artifacts. Here’s a picture of what the mesh looks like in that area.
Try selecting all of the vertices for your joined skull in edit mode, and recalculating the normals by hitting ctrl + n. Hope this helps. 
