Complex sentence: Pieces of mosaics and some columns were found early in the 19th century, and some excavations were carried out later in that century, but the first serious excavations were performed by Paolo Orsi in 1929, and later by Giuseppe Cultrera in 1935-39.
Split into simpler sentences: Pieces of mosaics and some columns were found early in the 19th century, and some excavations were carried out later in that century. The first serious excavations were performed by Paolo Orsi in 1929, and by Giuseppe Cultrera in 1935-39.