The facial ridge continues in a great variety of shapes, short or long, but always forming the fraction of a circle, always of nearly equal thickness, only slightly tapering towards one of the extremities, and always when in direct connection with the glabella, starting from the base of its foremost, largest segment. As a rule the ridge is more developed in the young or larval individuals, continuing from the glabella to near the posterior cephalic border in an uninterrupted arch[13] quite as in several of the adult Olenellidæ of the oldest Cambrian. It can be taken as granted that its origin is the same as in the Olenellidæ though at present the only evidence at hand is the small larva of Paradoxides oelandicus, which LINNARSSON called Parad. aculeatus.[14] In this we see the anterior pleuron or the facial ridge alone present, elongated downwards like the same pleuron in the figure 6 of Olenellus asaphoides and terminating like this in a fine spine stretching backwards outside the posterior border. Of the second pair of pleura there is nothing to be seen. This must then have been aborted at an earlier stage than in the Olenellidæ. [Footnote 13: G. F. MATTHEW had before me, as I now find, pointed out this distinction in his memoir »Illustrations of the Fauna of St. John» N:o IV, p. 163. When he speaks of »the embryonic stage» in this and other passages he evidently means »larval stages», as the embryonic stages of necessity must remain unknown to us. (Later remark.)] [Footnote 14: Om faunan i lagren med Paradoxides ölandicus (1877), p. 359, pl. 14, f. 11.] The connexion between the free cheeks and the middle part of the head has been very lax not only in the Paradoxidæ, but on the whole in nearly all Cambrian trilobites with free cheeks. When the free cheek is dissevered it shows no trace of the ridge, there is only a large scallop on the spot where it embraced the ridge. In the Paradoxidæ the rim of the indenture and the ridge are in so close contact that there is not the least place for an eye between them, as can be seen in the few specimens with a complete head. In all oculate trilobites again without any exception the facial suture separates that part of the eye which is the real visual organ with corneal facets, from the interior often elevated portion, opposite it, the so called palpebral lobe. The eye is _always_ placed on the free cheek,[15] the lobe again always on the fixed cheek of the head shield. No real eye exists without the palpebral lobe, and, on the other hand, that part of the facial ridge which later in the development changes to a palpebral lobe, occurs alone without any eye in a great number of Cambrian trilobites, and consequently these are blind and such is the case with the Paradoxidæ and a great number of the succeeding. [Footnote 15: Excepting in Harpes, which has no free cheeks.]