These look great for the most part. Sometimes you seem to take the form of the ribcage too small, like the second one on the second page. Always remember that the top oval includes the area of the ribcage as well as some of the oblique. It will be quite big even when foreshortened.

I would suggest using a tool besides a pen, such as a colored pencil. Try and copy the way Stan uses his pencil and you will see how much neater and more fluid your lines will come out. Hope that helps!