A Monograph of the Trilobites of North America

The head of the trilobite is also generally divided into three parts: the middle is called the front, or forehead; and the lateral portions the cheeks. In most cases, a projecting tubercle, or knob, is observable on the anterior surface of each cheek, which has much the appearance of an eye. Its reticulated structure is in many instances so analogous to that of the eyes of some crustaceous animals, and also of some species of insects, that there can be but little doubt that these tubercular projections, were true organs of vision.