Another group member points my to a Steven Saylor interview at Historical Boys, where Saylor refers to Livy.
Here is Livy, Vol. 1, Section 1.7
[…] Hercules grasped Evander's right hand and said that he took the omen to
himself and would fulfil the prophecy by building and consecrating the
altar. Then a heifer of conspicuous beauty was taken from the herd, and
the first sacrifice was offered; the Potitii and Pinarii, the two
principal families in those parts, were invited by Hercules to assist
in the sacrifice and at the feast which followed. It so happened that
the Potitii were present at the appointed time, and the entrails were
placed before them; the Pinarii arrived after these were consumed and
came in for the rest of the banquet. It became a permanent institution
from that time, that as long as the family of the Pinarii survived they
should not eat of the entrails of the victims. The Potitii, after being
instructed by Evander, presided over that rite for many ages, until
they handed over this ministerial office to public servants after which
the whole race of the Potitii perished. This out of all foreign rites,
was the only one which Romulus adopted, as though he felt that an
immortality won through courage, of which this was the memorial, would
one day be his own reward.
(Electronic Text Center, University of Virginia Library)
