Confused about crytsal HC49US

[desertgreg]

I am looking at a reference schematic which uses a 25Mhz HC49US crystal.  In the schematic there are three pins on the HC49US.  When I look this device up in both mouser.com and digikey.com I find that it looks like an oval metal 'can' with two leads.  I've attached a picture that has a snip of the schematic and a picture of the device.  I need to integrate this into a design I'm working on but what are the three leads in the schematic?

Thanks for any help you can give me!

[Vgkid]

The third pin is connected to the case, in that picture.

[ataradov]

And there is no real evidence that connecting the case to ground makes a difference. So if case does not explicitly has a pin for that, don't do it and skip this connection, it all will work.

[desertgreg]

Thanks that helped a lot!