Caracalla AE20 of Cassandreia, Macedonia.  

obv: M AVR ANTONINVS, laureate head right 
rev: COLO IVL AVG CAS, horned head of Zeus Ammon. 

SNG Copenhagen 154.

Reverse inscription is short for "Colonia Julia Augusta Cassandreia".
Cassandreia is the same city as Potidaia, and was renamed after Cassander
rebuilt it.  

See:

http://www.bartleby.com/65/po/Potidaea.html
http://www.whoosh.org/issue18/dickson1.html#Potidaea
Image contributed by "Ioannis Xanthopoulos", February, 2003.