In this way, perhaps, the loss of much of the external armature and of the two fore feet may be accounted for. The hind limbs, being stronger, were better able to resist such attacks, and they are therefore preserved. Like many other specimens, this fossil has, in the course of ages, been subjected to enormous pressure from overlying strata, causing compression and dislocation or fracture. [20] From Greek--_scelis_, limb, and _sauros_, lizard. But there were in existence during the long Jurassic period, other and even stranger forms of armoured Dinosaurs. One of these, only imperfectly known at present, was the many-spined Polacanthus.[21] This remarkable monster had the whole region of the loins and haunches protected by a continuous sheet of bony plate armour, rising into knobs and spines, after the fashion of the shield or carapace of certain extinct armadillos known as Glyptodonts (see Chapter XII.). A specimen of such a shield is to be seen in the collection at South Kensington (Wall-case 4). It is to be hoped that, some day, further remains of the Polacanthus will be brought to light, so that a restoration may become possible. Dr. Mantell had already pointed out certain analogies between Iguanodon and the huge extinct sloths of the South American continent, that flourished in the much more recent Pleistocene period; and this idea is now considerably strengthened by the later discoveries of armoured Dinosaurs. These are his words: "In fine, we have in the Iguanodon the type of the terrestrial herbivora which, in the remote epoch of the earth's physical history termed by geologists _the age of Reptiles_, occupied the same relative position in the scale of being, and fulfilled the same general purposes in the economy of nature, as the Mastodons, Mammoths, and Mylodons (extinct sloths) of the Tertiary period, and the existing pachyderms." [21] From Greek--_polus_, many, and _acantha_, spine.