One can observe that there as many pieces of the domino as free places in the snake. Moreover, lowercase Greek letters match uppercase ones. This means that domino is the right thing to try out. While building the domino (preferably from both ends), we can notice that "-" appears between pairs in rows while "_" between pairs in columns. But soon it seems there are more possibilities how to continue.

Now let's have a look at the Roman numbers. There are too big for any direct substitution for letters. However, we can subtract (-) or divide (_) them to get nice natural numbers ranging in 1...26.

This helps to finish the domino in a unique way and we can convert the resulting numbers to letters and read FERN BEHIND B EIGHT TWO.