strongly derived condition show the phalanges, small, disc-shaped elements positioned in long rows. Sometimes, the number of fingers is reduced, to as low as two. This is a rather common phenomenon within the Tetrapoda. Unique, however, for derived tetrapods, is the fact that some species show nonpathological polydactyly, the number of fingers being higher than five. Some species have ten fingers per hand. These fingers, again, can have an increased number of phalanges, up to thirty, a phenomenon called hyperphalangy, also known from the Plesiosauria, mosasaurs, and the Cetacea. The high number of elements allows the flipper to be shaped as a hydrofoil. When a high number of fingers is present, their identity is difficult to determine. It is usually assumed that fingers were added at both the front and at the rear, perhaps to a core of four original fingers. If fingers are added, often the number of metacarpals and carpals is also increased; sometimes even an extra lower arm element is present. Earlier, ichthyosaurs were commonly divided into "longipinnate" and "latipinnate" forms, according to the long or wide shape of the front flippers, but recent research has shown that these are not natural groups; ichthyosaur clades often contain species with and without elongated forelimbs. The ichthyosaur pelvis is typically rather reduced. The three pelvic bones: the ilium, the ischium, and the pubic bone, are not fused and often do not even touch each other. Also, the left and right pelvic sides no longer touch; only basal forms still have sacral ribs connecting the ilia to the vertebral column. The hip joint is not closed on the inside. The pubic bone typically does not connect to the ischium behind it; the space in between is by some workers identified as the fenestra thyreoidea; other researchers deny that the term is applicable given the general loose structure of the pelvis. Some later species have a connected pubic bone and ischium, but in this case, the femoral head no longer articulates with the hip joint. Triassic species have plate-like pubic bones and ischia; in later species these elements become elongated with a narrow shaft and can form a single rod. Typically, the hindlimbs are shorter than the forelimbs, possessing a lesser number of elements. Often, the rear flipper is only half the length of the front flipper. The thighbone is short and broad, often with a narrow waist and an expanded