Not unfrequently polydactylia is associated with the absence or union or decrease in size of bones of the limbs. It happens that the upper or lower extremities may be increased or diminished disproportionately through the body. This disproportion in size of the fingers and toes is exceedingly frequent. Big digits (macrodactylia, Fig. 99) are comparatively rare, and may only involve a supernumerary bone in the thumb. Short digits (microdactylia) are much more frequent. This state may be constituted by the absence of one bone or the union of two bones, or the shortening of metacarpal or metatarsal bones, or the shortness of several phalangeal bones. There may be increased disproportion between the different fingers. The method of determining this is by comparison with the middle finger. This disproportion may vary greatly. Féré is of opinion that shortening of all the fingers constitutes a grave mark of degeneracy. Relative shortness is exceedingly common. Under the conditions of development of the limbs from the fin-fold, it follows that these may be checked completely so that the condition approximates the earlier development from the fish. On the other hand the large bones of the arm and thigh may be checked while the digits and the two lower bones (radius and ulna, tibia and fibula) go on to full development as do the digits. Sometimes the arms develop completely while the lower extremity remains in the fin-fold state. On the other hand the arms may be checked and remain in the fin-fold state while the legs go on to full development. Sometimes the bones of the arm and forearm are checked while the digits go on to full development. The lower extremities are sometimes fused together. This condition, from its resemblance to the like state in the seal, is called phocomelia, or seal limbs. They are also called sirens, on account of the resemblance to the sirens of mythology.[227] [Illustration: FIG. 100.]