Definition of "Somersetshire"

In that part of the western division of this kingdom which is commonly called Somersetshire, there lately lived, and perhaps lives still, a gentleman whose name was Allworthy […]

1749, Henry Fielding, The History of Tom Jones, a Foundling

What have Hampshire and the contiguous areas got that Gloucestershire, Somersetshire and Wiltshire have not?

1962 April, “Death from Natural Causes?”, in Modern Railways, page 218