Some persistent archaic genera as Conocoryphe and Elyx inform us how this ridge has originated and how widely different it is from that in the Olenellidæ. On the surface of their head shield no ridge is visible, but on the interior side of it we behold, as the figures show (Pl. VI, figs. 43 & 44) on each side of the glabella a ramifying system as of the most minute vessels, which spread and cover the whole surface of the cheeks. This reticulation issues from a main trunk that goes nearly straight in an oblique direction from the foremost segment of the glabella and emits narrow branches forwards and backwards on its both sides. And these branches go on dividing till they occupy the entire interior surface of the fixed cheeks. The figures representing this are taken from Elyx laticeps fig. 43, and an almost similar image has been obtained from a species of Conocoryphe fig. 44. Now the question may be asked, what does this network of branching canals mean? I think we cannot gain a more plausible answer than that given us through the inspection of the soft parts that lie hidden behind the glabella of Limulus and its fixed cheeks, the only living crustacean which offers the greatest homologies with the head shield of the trilobites. Next below the shell of the glabella and the cheeks of Limulus there is a complicated stratum of muscles and behind this the heart and the great central circulatory system spreading from that centrum towards the sides of the head shield, the vessels being the more fine and minute, the more they are elongated from the centre. I now suppose that likewise in these trilobites the narrow and prominent glabella has been the receptacle not only of a strong mass of muscles, but also for the central part of the circulation. This centre has there sent out two relatively strong ducts or canals, one on each side of the glabella feeding all soft tissues inside and near the cheeks and probably also other important parts of the body. It is also clearly seen in Elyx how the orifice of the main trunc opens in the hollow of the glabellar apex.