Complex sentence: The matrix is made up from freshwater gastropods, "Viviparus" winkles, similar to but larger than those making Purbeck Marble-with the pale calcified remains of the shells in a matrix of darker material.
Split into simpler sentences: The matrix is made up of the shells of freshwater gastropods and "viviparus" winkles, similar to but larger than those making Purbeck Marble. The pale calcified remains of the shells are in a matrix of darker material.