What is this symbol

What is the device in the red circle to the left of the Schmitt trigger? This is from the ATMEGA328P datasheet.

Thank you, Dave

I have always assumed it is a bi-directional buffer - but I may be wrong.

...R

"transmission gate." Sort of like the output stage of a DC solid state relay; current can flow through it in either direction. Aka "analog switch." Here's an explanation: http://www.maximintegrated.com/app-notes/index.mvp/id/4243

Ah so! Thank you!