The Gastropods ➚ are a large group that include modern day snails are a very ancient group, this single division of the huge range of shellfish range from the earliest known shell fossils in the Cambrian of 600 million years ago to the present day. Compare this to our own Primate order which has existed for about the last 50 million years.
The Platyceras haliotis gastropod has the typical spiral form (helical spires) but in this case it is severely flattened. It is believed to have fed on crinoids because it is commonly found in association with these fossils. It is known from Silurian rocks.
Normally only the shell is preserved and as taxonomic classification is based on the internal structure of the living parts (the soft internal parts) this makes proper classification difficult.