In all crustaceans, as far as is now known, their more or less hard calcareous or chitinous skeleton is moulded by the subjacent tissues and glands. The sculpture of the surface consequently is an outcome of the fashioning procedures showing what has been going on below and what is still going on. If we then on the exterior surface of the head shield of the trilobites see the radiating lines in relief we must conclude that they are due to a subjacent system of almost capillary vessels causing hollow impressions on the inside and elevated ridges exteriorly. In Elyx the vessels have made no strong impression as to be visible on the outside. In a couple of American Conocoryphæ again (C. reticulata and C. trilineata) the main trunc of the vessels has formed a short faintly elevated ridge. In Solenopleura (Pl. VI, f. 45) the facial ridge has been fully developed, and by the casts of the inside it is clearly seen that the main trunc of the vessels makes the inside of the ridge and has been much incrassated. On the other hand the smaller branches issuing from it have been reduced in size and number, but are still visible. Thus, if I am right, the two, the facial ridge and the main trunc, are in causal connection and the former has been moulded by the later, when it had gained in size sufficiently, and when a richer affluxion of nutritive fluid was directed backwards towards the point where the eye at last originated. The further changes in this ridge thus are related to the development of the eye. As far as I am aware, there are as yet no data to tell us whether the appendages or pleura of the glabella in the larval (or primordial) Olenellidæ have the same origin as the ridge of the Olenidæ or are homologous with it. Perhaps it may be so in respect to the foremost one, which also is persisting. It is, however, not known how the pleura in the olenellid glabella have been developed, whereas it is well ascertained by the growth of the glabella in Sao, Liostracus and others that such pleura never have been developed in them. What the phylogenetic evolution has taught us concerning the formation of the facial ridge is confirmed by the ontogenetic development of individuals of some species as for instance Sao hirsuta BARR., Liostracus sp. BRÖG. and others.