The corresponding bone in the posterior fin/limb is the fibula.

http://upload.wikimedia.org/wikipedia/commons/6/67/Illu_upper_extremity.jpg

One of the two long bones of endochondral origin of the fore-epipodium; it is located on the medial (inner) side of the radius. The upper end of the ulna articulates with the lower end of the humerus forming a strong hinge joint with the carpals, whereas the lower articulates with the carpals.[AAO]

uberon

ulnar

UBERON:0001424

• radio-ulnar joint
• humeroulnar joint

• secondary remex feather

• radio-ulna

• extensor pollicis longus muscle
• supinator muscle
• abductor pollicis longus

• ulnare

• ulna pre-cartilage condensation

• lateral tuber of ulna
• diaphysis of ulna
• capitulum ulnae
• epiphysis of ulna
• ulnar tuberosity
• humeral facet on the ulna