This large division embraces the second and third groups or, with a few exceptions, all the rest of the Cambrian trilobites on account of a feature in the cephalic sculpture common to them all, though widely different as to its first origin in both. What forms the prominent and common characteristic of these two groups is the presence of the _facial ridge_, which emanates from the basis or the front of the foremost segment of the glabella and in a great variety of different shapes continues backwards near to the posterior border of the head. It has received several names as _eye-line, palpebral lobe, ocular ridge, eye-lobe, ocular fillet_ (MATTHEW). In German it is named _Augen-leiste_, in French _filet_ (BARRANDE) and in Swedish _ögonlist_.[4] Some authors make a difference between the more narrow part, calling it eye line, and the thicker posterior node, which they name the palpebral lobe proper. [Footnote 4: That name is the most current amongst the swedish authors, together with »_palpebrallob_»; HOLM says _ögonlob_ and _frontallob_.] As this peculiar ridge exists before any facial suture has made its appearance and separated the head shield in five parts, viz. the median glabellar part, the two fixed cheeks and the two free cheeks, and as it occurs in genera which never possessed any facial suture, and where no eye ever was formed, it is not adequate to call it an ocular ridge etc. the more so, as it, at least during a long series of genera succeeding each other, has had no connection whatever with any eye. I therefore propose to call it _facial ridge_ (in swedish _faciallist_). It occurs on the head of almost all Cambrian trilobites, excepting the archaic ones, and it is retained in the later Cambrian Peltura, Sphærophthalmus etc., which have real, compound eyes, as well as in a few Lower Silurian genera as Triarthrus, Pliomera,[5] Euloma, in the Upper Silurian Arethusina and Acidaspis and most persisting in Harpes, ranging from the Lower Silurian Lower Red Orthoceratite Limestone into the middle Devonian beds. [Footnote 5: Pl. Törnquisti HOLM.] It presents itself in the most variable shapes, and as it in fact can be followed through its development in the oldest known species, it is suitable to begin its description together with the characteristics peculiar to the oldest or second group in this large subdivision, that of the _Olenellidæ_. =Group 1. The Olenellidæ.=