Standard surface material presets missing

[cathy_easthope]

Hello.

An embarassing question, as I'm supposed to not be a beginner, but quite a few of my Arnold standard surface presets are missing (see the jpeg for the ones I do have). I'm running Arnold 4.2.2  in Maya 2020 on a Macbook Pro.
How do I get them back?

[Stephen.Blair]

That's a Maya Standard Surface shader, not the Arnold Standard Surface shader.