When the wombat is feeding, the two pairs of incisors grasp the grass stems, usually pulling them free without cutting them. The stems are then arranged in the diastema by the lips and tongue so that their ends are passed on to the cheek teeth, which consist of one premolar and four molars on each side of each jaw. These molar teeth have a flat surface, unlike those of other grazing animals. A kangaroo, for example, has two sharp ridges across the crown of each molar, which cut the grass stems as the lower molars are moved from side to side across the upper molars. In a study of the mastication of grasses by wombats, Kim Molina and Gordon Sanson found that the grass stems are reduced to small fragments in another way. There is a single sharp ridge of enamel on the cheek (labial) side of the lower molars and on the tongue (lingual) side of the upper molars. This ridge forms because the other side of each set of molars is free of enamel and therefore, being softer, wears away more quickly, leaving a sharp cutting edge on the opposite, enamelled side. The grass stems are first ground between the flattened crowns as the molars move sideways against each other and are then cut by the sharp enamel ridges as the lower ridge passes the upper one. This is a most efficient way of breaking up the tough, fibrous grasses that are the wombat's dominant food. Because considerable force is applied to the molar surfaces as they are ground against each other, they are constantly worn down, but this is overcome by their continuous growth.
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