Their commonest varieties are derived from Celtic prototypes and hardly occur abroad.

The most striking example of this is supplied by the enamelled 'dragon-brooches'.

Both their design (Fig.

19) and their gorgeous colouring are Celtic in spirit; they occur not seldom in Britain; on the Continent only four instances have been recorded.[2] Here certainly Roman Britain is more Celtic than Gallia Belgica or the Rhine Valley.
Yet a complete survey of the brooches used in Roman Britain would show a large number of types which were equally common in Britain and on the Continent.

Exceptions are always more interesting than rules--even in grammar.