Splitting the black shale of the Collingswood member of the Whitby formation I've found a few examples of what is likely a gastropod. The slab below shows what may be an example of Ecculiomphalus sp. or possibly Liospira sp. Both of those species are low conispiraled gastropods although Ecculiomphalus sp. should have some fine ornamentation on the shell. I don't see that on this specimen but William Hessin states that it is only found in the lower part of the Whitby formation (Craigleith member which is equivalent to the Collingswood member).
There is also a cast of a Pelecypod present on this slab that I believe is a species of Ctenodonta
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. 137-138, 144 to help identify these