I personally have very thin bones, slim waist and have been very skinny most of my life but I have really broad shoulders for size of my body. (fitnessandpower.com)
I would say I could be mesomorph with thin bones but there is no such thing according to these classifications. (fitnessandpower.com)
Upper Arm — muscular, of thin bone structure, as long as the shoulder blade; its inclination to the horizontal ranges from 58 to 60 degrees. (akc.org)