SALVS REIPVBLICAE

Bust of Galla Placidia to the right, draped with Chi-Rho on right shoulder, wearing a diadem of pearls, a double necklace of pearls and long earrings. 

Galla Placidia nimbate and enthroned facing, holding a mappa with the right hand and resting his feet on a footstool.

Photo with kind permission of the Bibliothèque Nationale de France, link to their wesbsite to see the coin: http://medaillesetantiques.bnf.fr/ws/catalogue/app/collection?vc=ePkH4LF7w1I9geonpBCEJmSF9MScnESFgpzE5MyUzERw9QrzDgAp4xci.

Coin with a large circular gold mount (for the one illustrated, no information on the second copy). Illustrated in RIC X plate 48 n ° 2009. Two known copies found in the Velp treasury. According to several articles, the denomination of the coin is 1.5 Solidus. The weight or diameter references that can be found for the two known medallions are: 24.00 mm for both (average?) and two weights: 31.40 grs and 40.00 grs (weight with the ornament, no possibility of knowing the weight of the medallion alone ). The two coins are now preserved at the bnf and Dutch National Collection.. The Bibliothèque Nationale de Francewho bought this medallion on January 16, 1883 at Rollin et Feuardent, gives a diameter of 5 cm (ornament included) and a more precise weight of 39.39 grs. 

I note that an illustration is visible in the Tanini: ''Numismatum imperatorum Romanorum a Trajano Decio ad Constantinum Draconem, Supplementum, 1791.''; plate VIII. 

The Chi-Rho on the shoulder seems to be the constantinian Chi-Rho.