Tiny

[Rockwood]

This was found in glacial material near Presque Isle, Maine. I believe it to be an internal mold of a brachiopod shell. 

What was it that lived in, on, or with it ?  Scale in mm.

[westcoast]

Sponge borings?

[TqB]

Ctenostome bryozoan boring?

As in this (second photo downwards):

https://woostergeologists.scotblogs.wooster.edu/2014/03/16/woosters-fossil-of-the-week-bryozoan-bored-and-bryozoan-boring-in-the-upper-ordovician-of-indiana/

[grandpa]

Gastropod feeding tracks?

[Rockwood]

These suggestions seem to imply that some of the actual shell remains. Notice the gap to the top and left. I assumed this meant it was a mold.

Perhaps a layer of the shell is more resistant to erosion ? 

[Mark Kmiecik]

7 hours ago, TqB said:

Ctenostome bryozoan boring?

As in this (second photo downwards):

https://woostergeologists.scotblogs.wooster.edu/2014/03/16/woosters-fossil-of-the-week-bryozoan-bored-and-bryozoan-boring-in-the-upper-ordovician-of-indiana/

This sure looks convincing -- nearly identical, and correct size-wise.

[TqB]

18 hours ago, Rockwood said:

Perhaps a layer of the shell is more resistant to erosion ? 

That's the impression the photo gives anyway. The small holes could be punctae.

[Rockwood]

The size and uniformity of shape make me think it is most likely a bryozoan.