The Carmarthen brooch, datable not later than c.a.d. 50 and perhaps as early as c. 25, is a direct two-piece successor to the pre-Roman, one-piece, Aylesford type, now known to be considerably later in date than used to be thought. The two-piece construction, which characterizes the entire British trumpet-brooch series, was adopted as a convenience when the bow was given a massive form in order to accommodate Late Celtic decoration in relief, of a western character and antecedents. The central moulding and foot carry petalled ornament of a ‘transitional’ or ‘supporting’ type: this ornament is reminiscent of that sometimes found between the circlet and terminals of early La Tène torcs abroad, but is probably better understood here as the carefully-designed continuation en suite of the open flower or rosette upon the headloop, and thus as half-open flower. Only the Carmarthen brooch retains such a headloop rosette.
The Carmarthen brooch was soon copied in bronze, and on these copies the relief appears in a coarsened and simplified form. The copies are entirely from western findspots. In the southern part of the region in which they occur, the medial petalling is retained (as at Lydney); but in the northern part, additional simplification has led to the suppression of the medial petalling (as at Segontium). Thus the Carmarthen brooch may be seen as the parent of the more fully-developed Rii class of Collingwood's typological sequence, and also of at least one strand in the Ri class, which has been considered up to now as the original form. In the west Midlands, perhaps seizing the opportunity offered by the simplified relief, craftsmen began to apply red enamel to the hollow areas, and to emphasize the central moulding, with its petalling, so that this became the principal plastic ornamental element (as at The Lunt, by a.d. 75). In the north of England, these novel features, enamel and exaggerated moulding, were further developed. The fine northern brooches (Backworth—not enamelled, but engraved—or Risingham) do not have headloop rosettes, and it was in the presence of their exaggerated central mouldings that Collingwood devised his celebrated ‘acanthus’ theory. By the Antonine period, brooches were being produced with polychrome enamel, and with their mouldings confined, very often, to the front of the bow; many of these have been found along the Rhine. Some late derivatives of the Carmarthen type are known from Wroxeter, and form a final group in the Appendix above.