Geisonoceras tenuistriatum cephalopod from the Whitby formation

I've found a couple of examples of small Cephalopods in the Collingswood member of the Whitby formation. They are hard to photograph as they do not contrast much with the black shale but I can see they are conical in shape and taper to a point. Some examples have fine ribbing or striations parallel to the shell growth. According to William Hessin these are likely Geisonoceras tenuistriatum which is restricted to the Whitby formation.

Specimen #1

Specimen #2

The Whitby 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. 155 to help identify these fossils.