Ammonite

Another rainy working day in front of the computer, so it's emergency blip time.

Time to raid the fossil cabinet. Here is a splendid fossil Cephalopod. An Ammonite, probably from the Jurassic with a well preserved original aragonite shell - many are just preserved as casts.

The shape of the shell shows slight lateral compression, a dense pattern of subdivided ribs and subinvolute coiling (coils are grown over and partially around the preceding whorl rather than just coiled around like a rope).

My best guess is that it may be a Pectinatid from the upper Kimmeridgian on the Dorset coast. Not having found it I don't know it's precise origin.