What have I started?

Looks awesome, as you can see: Progress!
You have the three skulls crossed out in the corner, which I presume you did first, then everything else has better more realistic proportions. :3 It's a matter of 'drawing what you see', for instance you drew in zygomatic ("Cheek") bone, and it's processs (The thin 'fence' of bone behind the cheek that goes along the 'temporal' ie side of the skull bone.)

Things that may help is if you use construction lines, to help line up the features with each other. So you can work in passes; first pass is just putting down geometric shapes like circles for eyes, a cynildral for the arch of the mouth to the chin, and you'd chistle and define these as you go onto the neck pass, but first it's just the placement and proportion.

Another thing is to do a 'thinner' outline, so it's not dark around the edges, and use the tones of the pencil to help solidify it as an object. (Difficult!) It's something my life drawing tutor told me about eyes, how doing thick outlines for eyes make them 'Disney-esque' and kills the degree of realism that would've made them insanely awesome otherwise.

Keep up the good work!

Thanks! Those first three were from a tutorial that I abandoned to draw purely from reference. And you're right; with each skull I drew, how the bones interlock became clearer and more intuitive. I don't have much knowledge of human anatomy, so it's good to learn the basics by drawing it.

I initially started learning the classical style of life drawing: draw the contour, then the large details, then work down from there. Since I ultimately want to draw in a style more suited to comics, I want to learn more about construction from geometric shapes. You can kind of see here that I started with a circle to define the shape of the skull, then I moved on to defining the chin and jaw to get proportions correct. Do you have any advice to make this easier?

And yes, shading is still something I need to practice. It still looks very blocky, jerky, and not very convincing. I've been trying to practice line weight to define the edges of an object, again with the goal of a comicbook style. I've wondered if I've been trying to draw too small, too--I feel like a lot of detail that would make my drawings more realistic gets lost.

For advice to make it easier; You already know about Contours, and we've mentioned we can construct it of shapes, but something that's pretty important is negative space: http://ctrlpaint.com/videos/drawing.....negative-space (Not the video I wanted but an informative one nonethless) So the space that makes up the interior darkness of the nasal passage, to the interworkings of the jaw's hinge, and the space between teh teeth and the hinge; focusing on those areas where there is 'nothing' can help give it form, as it's easier to see that absence of 'what is not there' than trying to look for the start or end of something that is and figuring out where to draw that.

Another thing is the 'basic unit' looking for some shape or feature in any life object, that can be used for a quick comparision, if you saw the skull at an angle, and it looked as if the space of the eye socket looked twice as tall as the forehead that's something you can pay attention to, and you use this 'basic unit' to measure other pieces around the drawing, rather than working out from different areas expanding out. (If you see someone holding out a pencil before they draw, they're looking for the basic unit/checking proportions.)

For working with tones; make sure you have pencils in different thickness of lead (the thickness refers to the concentration of graphite than the pencils actual width of marks, so B to B6 is really dense and it shreds easily thus leaving darker marks, which really stand out) and if in doubt on how dark an area is, just squint. You'll see everything blurred, but can make out the light and dark areas and it gives you a better idea on what you need to do.

You can work bigger if you feel you need to, the details I really want to add more is the sutures of the skull, where the plates of bone fuse together. I just think it makes it look less plastic and more Organic and natural, seeing it comprised of the different shapes fitting together.

Good things to note, thank you!

I've already invested in a set of pencils of varying thickness, but I find myself using a 2B for pretty much everything.

Same, 2B is a good all-rounder. But if you get the chance you could try using willow charcoal sticks, but you need paper with a rough texture. Charcoal studies is something that's common to Art School Life Drawing Classes, but I need to put on headphones as I can't stand the sound it makes. >.O; Same with Pastels or any chalky medium.

But one thing that comes out of it, is you can use a eraser to add in highlights. Which will work for pencil sketches as well. We had received a hard edged rubber that was very firm for sharp edges, while we got this kneaded eraser to break off into smaller bits and roll it like putty, to give a gentle gradations. I think that's all the advice I can give for now.