A Pterodactyle's teeth vary a good deal in appearance. The few large teeth in the front of the jaw in Dimorphodon, associated with the many small vertical teeth placed further backward, suggest that the taking of food may have been a process requiring leisure, since the hinder teeth adapted to mincing the animal's meat are extremely small. The way in which the teeth are shaped and arranged differs with the genera. In Pterodactylus they are short and broad and few, placed for the most part towards the front of the jaws. Their lancet-shaped form indicates a shear-like action adapted to dividing flesh. In the associated genus Rhamphorhynchus the teeth are absent from the extremity of the jaw, are slender, pointed, spaced far apart, and extend far backward. When the jaws of the Rhamphorhynchus are brought together there is always a gap between them in front, which has led to belief that the teeth were replaced by some kind of horny armature which has perished. In the long-nosed English type of Ornithocheirus the jaws are compressed together, so that the teeth of the opposite sides are parallel to each other, with the margins well filled with teeth, which are never in close contact, though occasionally closer and larger in front, in some of the forms with thick truncated snouts. It is not the least interesting circumstance of the dentition of Pterodactyles that, associated in the same deposits with these most recent genera with teeth powerfully developed, there is a genus named Ornithostoma from the resemblance of its mouth to that of a bird in being entirely devoid of teeth. It is scarcely possible to distinguish the remains of the toothed and toothless skeletons except in the dentary character of the jaws. There is no evidence that the toothless types ever possessed a tooth of any sort. They were first found in fragments in England in the Cambridge Greensand, but were afterwards met with in great abundance in the Chalk of Kansas, where the same animals were named Pteranodon. A jaw so entirely bird-like suggests that the digestive organs of Pterodactyles may in such toothless forms at least have been characterised by a gizzard, which is so distinctive of Birds. The absence of teeth in the Great Ant-eater and some other allied Mammals has transferred the function which teeth usually perform to the stomach, one part of which becomes greatly thickened and muscular, adapting itself to the work which it has to perform. It is probable that the gizzard may be developed in relation to the necessities which food creates, since even Trout, feeding on the shell-fish in some Irish lochs, acquire such a thickened muscular stomach, and a like modification is recorded in other fishes as produced by food.