This is a placenta with two umbilical cords. One baby was born normal. This baby died in the middle of the pregnancy.
How many amnionic sacs do you see? If twins share an amnion, then they are probably identical.
There appears to be two amnions, the one on the right looks as if it has twisted over the left amnion. Since the twins were probably not identicle, this may have been the factor that allowed one twin to survive; since the stillborn may have died due to a chromosomal abnormality.