One of the more common species of Brachiopod in the upper Ordovician is Ryhnchotrema. This genus of Ryhnchonellids are generally small and roughly triangular in shape with little to no hinge line. Two species are found in the Coburg formation, R. capax and R. increbescens. Their shell shape is the only easy way to tell them apart as both species have a sulcus with three folds, both have ornamental concentric growth lines, and both can be found in the same layers of the Coburn formation.
R. capax has a more rounded or rectangular form while R. increbescens is very triangular and ends with a sharp beak on the pedicle valve.
The Coburg formation is Ordovician in age (late Ordovician, Katian/Mohawkian stage).
These fossils were collected from the St. Mary's Limestone Quarry in Bowmanville, Ontario. I used William Hessin's book "South Central Ontario Fossils", 2009, Self Published, pg. 130 to help identify these fossils.