In the black oil shales of the Collingswood member of the Whitby formation there are often found great shoals of Ostracod fossils.William Hessin states in his book that the only known species of Ostracod found in these shales is Primitiella ulrichi. The fossils are plain and oval shaped, kinda boring really and easily overlooked except for their numbers.
Here are two pieces of shale that shows how one typically notices them.
Under the microscope is where you can see them up close. Rarely are the shells still present as they often flake off when the shale splits. All of the pictures below were taken with my camera through a microscope.
The Whitby formation is Ordovician in age (late Ordovician, Katian/Mohawkian stage).
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. 211 to help identify these