Turning to the Pterodactyles themselves, Von Meyer divided them naturally into short-tailed and long-tailed. The short-tailed indicated by the name Pterodactylus he further divided into long-nosed and short-nosed. The short-nosed genus has since been named Ptenodracon (Fig. 59, p. 167). The long-tailed group was divided into two types--the Rhamphorhynchus of the Solenhofen Slate (Fig. 56, p. 161) and the English form now known as Dimorphodon (Fig. 52, p. 150), which had been described from the Lias. The Cretaceous Pterodactyles form a distinct family. So that, believing the tail to have been short in that group (Fig. 58), there are two long-tailed as well as two short-tailed families, which were defined from their typical genera Pterodactylus, Ornithocheirus, Rhamphorhynchus, and Dimorphodon. The differences in structure which these animals present are, first: the big-headed forms from the Lias like Dimorphodon, agree with the Rhamphorhynchus type from Solenhofen in having a vacuity in the skull defined by bone, placed between the orbit of the eye and the nostril. With those characters are correlated the comparatively short bones which correspond to the back of the hand termed metacarpals, and the tail is long, and stiffened down its length with ossified tendons. These characters separate Ornithosaurs with long tails from those with short tails. The short-tailed types represented by Pterodactylus and Ornithocheirus have no distinct antorbital vacuity in the skull defined by bone. The metacarpal bones of the middle hand are exceptionally elongated, and the tail, which was flexible in both, appears to have been short. These differences in the skeleton warrant a primary division of flying reptiles into two principal groups. The short-tailed group, which was recognised by De Blainville as intermediate between Birds and Reptiles, may take the name Pterodactylia, which he suggested as a convenient, distinctive name. It may probably be inconvenient to enlarge its significance to comprise not only the true Pterodactyles originally defined as Pterosauria, but the newer Ornithostoma and Ornithocheirus which have been grouped as Ornithocheiroidea. The second order, in which the wing membrane appears to have had a much greater extent, in being carried down the hind limbs, where the outermost digit and metatarsal are modified for its support, has been named Pterodermata, to include the types which are arranged around Rhamphorhynchus and Dimorphodon.