Arugula is very salty, and doesn't look like arugula
Round-leaf arugula is apparently an actual variety that exists.  However, it is not noted for having a particularly salty flavor.
Given the strong salty flavor, and the appearance, it's more likely that you got orach instead.  A nutritious and tasty green, Orach is known for its high salt content, making it excellent in salads (just don't add more salt to it).