There are lots of Flat Periwinkles (Littorina obtusata Linnaeus) grazing on the seaweeds at Kimmeridge Bay. I got down really low to look this one in its beady black eye as it felt its way forward with antennae over a frond of Toothed Wrack. Both the shell and the body are yellow but shells can range from white to dark brown. This small gastropod mollusc is common on most British rocky shores.
For more information about Flat Periwinkles have a look at the Marine Life Information Network site run by the Marine Biological Association of Great Britain.
For more information about Kimmeridge Bay see the Purbeck Marine Wildlife Reserve Web Site.