A theory of the development of mammalian molar teeth. The primitive stage is that of simple cones, as in reptiles. The simple cone then developed a smaller cone in front and another behind. Next, a cingulum was developed, and the three cones became arranged in a triangle, the two smaller cusps having moved to the outer side in upper and to the inner in lower molars. This primitive triangle is called the trigon or trigonid and this stage the tritubercular or trigonodont. The trigon being a cutting apparatus, an extension of the posterior part of the crown was developed in lower molars for crushing, and a smaller corresponding part appeared in upper molars. Another large cone then arose, usually from the cingulum. In more complex forms, smaller intermediate cusps appeared.
