Complex sentence: The "Carmen Arvale" is preserved in an inscription of AD 218 which contains records of the meetings of the Arval Brethren, but is in a more archaic stage of Old Latin, likely not fully understood any more at the time the inscription was made.
Split into simpler sentences: The "Carmen Arvale" is preserved in an inscription dating from 218 AD which contains records of the meetings of the Arval Brethren. It is written in an archaic form of Old Latin, likely not fully understood any more at the time the inscription was made.