It is remarkable that some of the Conocoryphidæ have an imperfect facial ridge, to be compared with the commenced one in Sao Barr. I (pl. 7 fig. 9). So the American Con. trilineata and reticulate. Walcott U. S. Geol. Survey 10th Rep. pt. I, pl. XCV f. 5 & 6. It is, as it were, arrested in the development and these adult trilobites had stopped, where the larva of Sao was proceeding in its second stage. They are the forerunners of the blind trilobites with facial suture, belonging to the third group. It needs scarcely be mentioned that the genera now enumerated have hardly anything in common, beside the general character of the head, and that real affinity exists only between Agnostus and Microdiscus, and probably also between Conocoryphe, Ctenocephalus and Elyx.
Beecher[2] asserts that there is a suture in Agnostus, but in vain we have searched for it in numerous well preserved specimens and Dr Holm also denies its presence. Nor are there any signs of closed up sutures, which also could not possibly be expected in so early a stage of evolution. It may then be taken as well settled that a fundamental character in these the oldest[3] of all known trilobites is the total want of a facial suture and a compactness of the whole head shield which later is broken up in several parts through the disjunction of the free cheeks. In the Lower Silurian formation there are a few genera sharing in the same structure of the head shield, though by no means else related. Such are Dindymene, Areia, Carmon and in the U. Silurian Cromus. The two species forming Barrande's genus Dindymene are so dissimilar that Dind. Friderici Augusti had better to be removed to a new genus and the first described one to be retained as type of the genus Dindymene. The same is the case with Carmon, where the type species C. mutilus is blind and without free cheeks while the other species, known only by its fixed cheeks and glabella is one of the Olenidæ.
[2] Nat. Classification of the Trilobites, p. 183.
[3] Oldest in that sense that they are the descendants of an archaic precambrian stock, the chief characteristics of which they have retained in the main unchanged and persisting long ages after the close of the Cambrian times, some, as Agnostus, continuing high up in the Lower Silurian.
II. Blind trilobites with facial ridge.
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.
[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.
[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.