That the recipient class is of later introduction than the received is evident from several considerations. In the first place, a flat blade not only approaches most nearly in form to the stone hatchets or celts which it was destined to supersede, but it also requires much less skill in casting than the blade provided with a socket. For casting the flat celts there was, indeed, no need of a mould formed of two pieces; a simple recess of the proper form cut in a stone, or formed in loam, being sufficient to give the shape to a flat blade of metal, which could be afterwards wrought into the finished form by hammering. And secondly, as will subsequently be seen, a gradual development can be traced from the flat celt, through those with flanges and wings, to the palstave form, with the wings hammered over so as to constitute two semicircular sockets, one on each side of the blade; while on certain of the socketed celts flanges precisely similar to those of the palstaves have been cast by way of ornament on the sides, and what was thus originally a necessity in construction has survived as a superfluous decoration. There is at least one instance known of the intermediate form between a palstave with pocket-like recesses on each side of a central plate and a celt with a single socket. In the museum at Trent[391] there is an instrument in which the socket is divided throughout its entire length into two compartments with a plate between, and, as Professor Strobel says, resembling a palstave with the wings on each side united so as to form a socket on each side. The evolution of the one type from the other is thus doubly apparent, and it is not a little remarkable that though palstaves with the wings bent over are, as has already been stated, of rare occurrence in the British Islands, yet socketed celts, having on their faces the curved wings in a more or less rudimentary condition, are by no means unfrequently found. The inference which may be drawn from this circumstance is that the discovery of the method of casting socketed celts was not made in Britain but in some other country, where the palstaves with the converging wings were abundant and in general use, and that the first socketed celts employed in this country, or those which served as patterns for the native bronze-founders, were imported from abroad. Although socketed celts, with distinct curved wings upon their faces, are probably the earliest of their class, yet it is impossible to say to how late a period the curved lines, which eventually became the representatives of the wings, may not have come down. This form of ornamentation was certainly in use at the same time as other forms, as we know from the hoards in which socketed celts of different patterns have been found together. As has already been recorded, the socketed form has also been frequently found associated with palstaves, especially with those of the looped variety.