In the British Museum, Greek and Roman Antiquities Department, this glass intaglio shows a snail under an ear of corn [1; “The term refers to stone or glass (or other materials) into which an image or text has been cut or incised. These texts or images are sometimes cut in reverse in order to produce a positive impression for use as a stamp or seal.”]. It is very small (12 mm diam.) and the snail is not so clearly visible, but definitely a sinistral shell. This may have been deliberately done in order to produce a ‘normal’ result when used as a stamp or seal.
It is from the Roman period, but lacks a more precise time indication.
 BML, inv. [GRA] 1814.0704.2075. http://bit.ly/1noDjiX.