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First molar - lower teeth
The first mandibular molar is the more constant in form. It has five cusps on the grinding surface. Four occupy the angles and are separated by a crucial fissure. The fifth cusp is situated at the posterior extremity of the longitudinal fissure. It has two roots, inclined somewhat backwards, placed one in front of the other. Each root is marked by a vertical groove, which is sometimes so deep that it divides it into two parts producing four roots in all. In some cases one root only divides in this way.