This is a loose brachial valve embedded in the matrix.
It displays imbricating strong growth lines very well. One of the keys, that I've now learned, is that if the imbricating growth lines only cover the front 1/3rd of the shell then the genera is Ryhncotrema.
Here is a loose specimen
Both specimens shown above were collected from the Verulam formation near Brechin, Ontario. The Verulam formation is Ordovician (Katian/Mohawkian) in age.