Is this correct?

What line of longitude forms the other half of the prime meridian?

When joined with the Prime Meridian the 180 degree of longitude forms a complete circle

Right.

No, that is not correct. The other half of the Prime Meridian, which runs through Greenwich, England, is the 180th line of longitude. The 180th line of longitude is also known as the International Date Line. It is where the date changes when crossing from east to west, and it roughly follows the 180th meridian of longitude.