For the rest the whole exterior surface is covered with terrace lines running concentrically and varying according to the different genera and even species. Many other genera, again, as Acidaspis, Calymmene, Homalonotus, Dalmanites etc., are devoid of them and instead granulated or smooth.
In the plurality of species there are two tiny patches or maculæ, sometimes elevated above the surrounding surface like tubercles and so they have also been called by some authors. But I have preferred to use the name »macula» for them, as the plurality does not form tubercles. They are generally entirely smooth and glossy and situated next to the anterior groove, either above it or in it, at a regular distance from each other and the lateral margins. They may form a sunk spot or, as commonly, an ovoid or elliptic area surrounded by a linear elevated border. Thus amongst the Asaphidæ. In others, again, (Bronteus, Illænus etc.) they form a moderately elevated tubercle. The direction in which they are oriented in relation to the longitudinal axis of the hypostoma is quite as variable. It is to the closer consideration of the nature of these macula that this memoir will be chiefly devoted. They have been delineated several times, but very seldom has any accurate attention been paid to them by previous authors. It is highly doubtful if Barrande ever alluded to them, when he in his »Système Silurien de Bohéme» vol. I, p. 156, in describing the hypostoma, says: »Cette plaque bombée porte souvent des saillies et des empreintes creuses, dans le voisinage de la bouche. Leurs formes varient suivant les espèces mais en conservant toujours les caractères génériques. Nous les considérons comme les points d'attaches des muscles et des màchoires.» In the numerous specific descriptions in his grand work there is never any mention made of the macula and in contemplating the figures and reading the explanations of these, we shall find that he meant something more comprehensive with his statement, to wit, the entire groove where these tubercular macula are situated.
In Novák's third paper, p. 4, we see the figures of two Phillipsiæ with tubercles on the hypostoma and he says only that in the anterior groove there are two such symmetrically placed ... glandular intumescences.[1]
[1] »In der Mittelfurche sitzen 2 kleine symmetrisch gelegene, nicht immer dentliche drüsenartige An schwellungen.»
Brögger again in his paper on the »Ausbildung des Hypostoms» devotes a page (p. 19) to explain the nature of these maculæ and regards them as the exterior marks of two muscular impressions or points of attachment on the inside for muscles which have attached the hypostoma to the walls of the glabella or the head. The real nature of these so variable tubercular maculæ is, however, a quite different one, as we hope to be able to demonstrate in describing all their varying shapes in several genera.
The interior surface of the hypostoma shows all the protuberances of the exterior surface (tubercular maculæ, spines and granulations) as shallow pits, the exterior grooves being on the contrary elevated ridges. For the rest the interior surface is smooth and its margins are posteriorly covered with a more or less broad duplicature, from the lateral margins of which the posterior wings project.
The manner in which Liljevall became aware of the interesting structural features of which I am going to give an account, is as follows. We were about to describe and delineate some new or not sufficiently known Upper Silurian trilobites from the island of Gotland. In order to draw a specimen of Bronteus polyactin Angelin, which is rarely found in the Wenlock and Ludlow strata of Gotland, Liljevall was preparing and cleaning its hypostoma. The shape of this hypostoma, as represented in [pl. II] fig. 22, is clypeiform, tapering towards the posterior margin, which is regularly tongue shaped. The anterior margin is faintly curved and at both sides elongated in a broad, pointed wing. Its exterior surface is divided in three transverse fields, defined through two shallow grooves, the anterior and largest field occupying more than the moiety, the median one is crescent-shaped and narrow, and the posterior one is nearly of the same size and confluent with the lateral borders of the hypostoma, which near the median transverse axis of the latter project in a blunt angle on each side. The surface, for the rest smooth, is covered by concentric, irregular terrace lines. On the inferior edge of the upper groove two small tubercles are situated, one on each side closer to the lateral margins than to the central axis. The size of their longitudinal axis amounts to 0,96 mm. in the shortest and to 1,07 mm. in the longest specimens. This axis is parallel with the groove and consequently oblique to the longitudinal axis of the hypostoma. They are oblong or ellipsoid, their inferior apices bluntly pointed or rounded, varying a little as seen in the three specimens figured, (Pl. II figs. 23, 24, 25). For about two thirds their surface is perfectly smooth or rather glossy, and the lower third is covered with a compact accumulation of small granules. The extension of this peculiar accumulation varies, being somewhat larger in some specimens, and the limit which runs oblique towards the glossy part is more or less curved. Such a granule taken single is of the extremest minuteness, measuring 0,055 millimeter at the highest. Their outline is perfectly circular and semiglobose.
When Liljevall saw this group of granules he was at once struck with the perfect, outward similarity between them and the facets of the compound eyes on the upper surface of the head of this same species, and I fully concurred in the same view. It now became of importance to ascertain whether the intimate structure also corresponded in both, and microscopic sections were accordingly prepared. Although not so clear as in other species of this genus a vertical section of a cephalic eye (Pl. II fig. 21) shows a median row of black spots which compared with other sections (Pl. II fig. 9) must be the centres of the badly preserved lenses. This is confirmed by the horizontal section (Pl. II fig. 20) where the well known image of the polyedric facets appeared. In the same manner the granulated spot of the hypostoma shows in a vertical section even more clearly ([pl. II] fig. 27) the elongate, coherent lenses with a black centre and the polyedric form of the lenses in a horizontal section (Pl. II fig. 26). The assumption of a pair of small adventive eyes on the exterior side of the hypostoma became thus based on the clear evidence of the perfect structural agreement between them and the eyes of the head. But it was necessary to strengthen this evidence through extended researches on the hypostoma of as many trilobite genera as possible and also to study the intimate structure of the cephalic eyes in as many genera as could be accessible.
We shall now give first a general account of the structure of the eyes in the trilobites as far as we have been able to study them, and then proceed to describe the maculæ of the various genera, which we have observed.
With regard to the presence or non existence of visual organs, we must remark that a considerably greater number of genera than those which unanimously have been regarded as blind, also are devoid of eyes though they still by many authors are ranked amongst the oculate ones and we then had better first to make a review of them and thus to eliminate them from the number of the oculate species.