Remember to have the cross-hatch lines follow the form. It might help to do perspective exercises drawing basic objects with a lot of lines on top - like computer wireframe models.
Or you can just copy renaissance drawings, then you'd also see different styles of cross-hatch as well as rendering.
Try to mark the corner of the jaw. The jaw might in reality be very curvy, but you can draw it with a very clear corner if you'd prefer. This would definitely help the perspective construction since boxes are easier to construct in perspective, and also much easier to render.
I find the point of the cheekbone at B to be a little lower then at A, this I do by holding up the pencil horizontally and looking on the model, then placing it horizontally on the paper.
Measuring like this I think is important.
Although I like to compare points using horizontal and vertical lines, I don't do this much if the points are in perspective, then I prefer to relate them to the established perspective. In this case all the points located on the front of the skull should be constructed using perspective lines rather than horizontal lines.
This line under the cheekbone I always give a strong line, it's a downplane and it should be clearly seen that it is infront of the jaw
This line is the change of direction of the plane of the forehead. Right now it's just a constructionline, depending on how much rendering I feel like doing I might very well just leave it like this.
You don't have to smudge much, often it's enough to just indicate both direction and value with cross-hatch lines.
Personally I like to do cross-hatch lines on top of a smudged/darkened area, I don't really know any rules with regards to cross-hatch, if in doubt it helps to study and do basic perspective exercises as well as copying renaissance prints or drawings. Again if the final results describes the form then you're doing it right! - Sometimes it helps to do parallel lines on general areas, disregarding smaller forms.
When using a black and white pencil I break down the line if it changes angle.
An example of rounded cross-hatch lines.
If you look at the drawings by Leaonardo da Vinci where he uses cross-hatch, then you'd find a lot of this type of round lines.
Especially in his anatomy studies of the old guy. Just google Leonardo + anatomy, or get the book Leonardo on anatomy(I think it's called).
A completely different style of cross-hatch is found in the drawings by Michelangelo(although most of those are student copies), he usually avoids round lines, instead uses straight lines with many layers on top of eachother. Some say they can find the same lines in his sculptures, by comparing pre-sketches of sculpts to the finished sculpture.
The best example of cross-hatch I know is found in this book :
"Albinus on anatomy" http://www.amazon.com/Albinus-Anatom.../dp/048625836X
You can just get the book "Drawing lessons from the great masters" by Robert Beverly Hale, as it includes many of Albinus' drawings.
What's so impressive is the tonal qualities he achieves with only lines.
(Actually Albinus didn't do the drawings, he was a doctor who hired an artist to present his anatomical studies)
Here I use straight lines, disregarding details, toning down the side plane.
I think the reason I tilt them is because the skull is tilted back. If just put down on the table like this it's actually tilted back compared to what it would be attached to the spinalcolumn.
At some point it might help to do what feels natural, the same is the case with ligth/shade. And it helps to be brave and experiment.
I indicate the plane change with simple cross-hatch lines. This is much faster than smudging to get a pure/even tone.
If it helps you can draw a super simplified geometric shape of the jaw, and continue it behind the teeth, it might help in understanding the 3D construction. If you do this remember to also use construction/perspective lines.
The red lines are the visible part of the jaw, the blue lines describes the jaw behind the teeth.
Sideplane of the jaw - the jaw is narrow in front and wide in the back.
It should be clear that this is a simplified construction - later you can round it with a soft transition, or leave it like this if you prefer.
For some reason I don't bother with a dark value on this sideplane - I believe the perspective doesn't leave any doubt that this is a sideplane.
For a short anatomy study I suggest you are a bit lazy, since it's much more helpful to do 10 of these from different angles, than just one finished "artwork". The point isn't to do art, the point is to learn the structure of the skull.
With that being said - if your goal is to practice rendering, or cross-hatch, then sure you can go on. In my experience it's better not to set up a time-limit when practicing rendering. I once redid the same part of a drawing 6 times before I was satisfied with the rendering.
This is an important detail of the back of the skull - because it's the attachment of the SCM muscle. Other than that I only care about the big planechanges of the back of the skull.
This white line is actually the frontplane of the sideplane of the jaw.
I don't really remember the anatomical term.
In this case all I have to do is to indicate it with a white line. So in a way it's both a line and a plane. It might be a bit exaggerrated but I don't mind.
This line groups the bottom of all the upper teeth, you always have to group details like this, before starting on individual teeth.
I keep the very high contrast transitions, if I had started on the teeth I definitely would have softened the transitions. Like this it gives a feeling of a very sharp angle change, that in a way is a bit far from the truth, or better, a simplified truth.