This is the raised gravestone marking the place of burial of Dom. Jacobus or James Wallace. This barely readable marker is in Latin, not surprising since Fr. Wallace was a Jesuit priest born in Kilkenny, Ireland about 1783. See translation below.
Latin scholars may want to closely examine the inscription (photo below). The fact that it is in Latin probably explains why the 1936 WPA folks didn’t try to copy it. Here is what they recorded:
According to “A History of St. Peter’s Church – 1824 – 1990,” Wallace was a South Carolina College professor of math and astronomy, famous in some circles for having published an astronomy book, “On the Globes.” Father Wallace accumulated some property during his life and left it all to the Church for establishment of a male orphanage in Charleston, SC.
There is an article in Catholic Miscellaney about Wallace: “Questions of ‘what if’ continue to mark life of 19th century Jesuit”
His book is described and is available for purchase on Amazon.
Below is a close-up photo of the inscription.
And this from the Diocese of South Carolina Archives: http://www.catholic-doc.org/archive1/index.php?p=collections/findingaid&id=370&q=&rootcontentid=70452#id70452