Hi Steve, welcome. I am not a mycologist, but usually a linguistic explanation fits, too.
You got an excipulum, which (usually) consists of more than one layer. The outside layer is called ectal, the inside layer medullary. Whilst it could happen, that all the layers are of the same kind, sometimes ectal and medullary layer are of different size/shape/composition, whatsoever. And then you can have gradual transition between them or a sharp one.
When you ask for ID, it would be always useful to indicate the location of the finding. mt - stands for Malta?
Best regards, Bernd
