Supernumerary paired structures are not usually, if ever, formed when an appendage is simply amputated. Cases occasionally are seen which nevertheless seem to be of this nature. Borradaile,[8] for example, described a crab (_Cancer pagurus_) having in place of the right chela three _small_ chelae arising from a common base, where the appearances suggested that the three reduced limbs replaced a single normal limb. From the details reported however it seems still possible that one of the chelae (that lettered F. I in Borradaile's figure) may be the normal one, and the other two an extra pair. The chela which I suspect to be the normal is in several respects deformed as well as being reduced in size, and this deformity may perhaps have ensued as a consequence of the same wound which excited the growth of the extra pair. Its reduced size may be due to the same injury, which may quite well have checked its growth to full proportions. Admitting doubt in these ambiguous cases it seems to be a general rule that for the production of the extra pair the normal limb should persist in connexion with the body. Moreover it is practically certain that in no case can a _single_, viz. an unpaired, duplicate of the normal appendage grow from it. Many examples have been described as of this nature, but all of them may be with confidence regarded as instances of a supernumerary pair in which only the two morphologically anterior or the two morphologically posterior surfaces are developed. We have thus the paradox that a limb of one side of the body, say the right, has in it the power to form a pair of limbs, right and left, as an outgrowth of itself, but cannot form a second left limb alone. A very interesting question arises whether it is strictly correct to describe the extra pair as a right and a left, or whether they are not rather two lefts or two rights of which one is reversed. This question did not occur to me when in former years I studied these subjects. It was suggested to me by Dr. Przibram. The answer might have an important bearing on biological mechanics, but I know no evidence from which the point can be determined with certainty. In order to decide this question it would be necessary to have cases in which the paired repetition affected a limb markedly differentiated on the two sides of the body, and of course the development of the extra parts in order to be decisive must be fairly complete. One example only is known to me which at all satisfies these requirements, that of the lobster's chela figured (after Van Beneden) in _Materials for the Study of Variation_, p. 531, Fig. 184, III.