Part 10

At first we must point out a certain resemblance which prevails between the hypostomas and maculæ in Bumastus and Bronteus. Compare for instance Bum. sulcatus with Br. platyactin and Br. laticauda. The general form is nearly the same in both and the shape of the maculæ almost identical but for the complete want of granulations on those of Bumastus. To this resemblance must be added that there are, as known, forms of trilobites which by some authors have been regarded as Brontei and by others, again, ranked amongst the Illænidæ.

Three species of Bumastus have been recorded as found in the Gotland strata, but of one of these no hypostoma is known. Besides, to judge by the head shields and hypostoma, there are a few new species. To begin with the most common

=B. sulcatus= LDM.

Pl. II figs. 33-40.

The hypostoma is broadly shield-formed and the anterior margin is most characteristic being in its central part elongated in a short evenly rounded projection. The side wings are triangular acuminate, bent a little backwards and towards the interior surface. There is only a single rounded field passing into an evenly bent border, in front of which the projecting, elliptic maculæ lie. A second pair of wings stretches backwards near the posterior margin. We have not obtained any sections of the maculæ.

The cephalic eyes figs. 36-38 are covered with an uncommonly thick stratum of homogenous shell, one and a half times as thick as the subjacent lenses. These are comparatively broad, above passing indistinctly in the covering stratum, below, at their basis, convex. In a horizontal section figs. 33, 34 they exhibit the common hexaedral or pentaedric aspect, their exterior sides being white and confluent without any separating lines. Their interior is dark and from the polygonal sides white lines go in giving the whole the appearance as of a starry, composite coral with its septa. The correspondence in structure with Bronteus laticauda pl. II fig. 7 is striking. A narrow space separates the surface of the eye from a nearly similar patch below the eve on the cheek 35 b, 38 b. But there the position of the composing strata is singularly reversed and the spongious or pseudo-prismatic stratum lies close to the outside and the homogenous stratum on the inside. The white longitudinal lines are well marked out and regular in the vertical section, fig. 38 b, indicating the prismatic structure most clearly.

=B. barriensis= MURCH.

Pl. II figs. 31, 32.

The hypostoma fig. 31 is of so transverse a shape that the breadth much surpasses the height, the former being 6 millim., while the latter attains only 3,5 millim. The anterior margin is so much arcuated and sloping backwards that it forms an obtuse angle. The side wings are triangular and acuminated and from them a thick pad runs backwards and forms the posterior margin. Below the largest field of the exterior surface the maculæ rest on a ridge having a much elongated, vermiform shape and sunk in the surface (fig. 32) a little more than three times longer than broad. In its shape it approaches much to the hypostoma of Bum. insignis HALL as delineated by SALTER pl. 27 fig. 7.

In another, undetermined Bumastus (pl. II figs. 47, 48) the hypostoma (from Klints, Othem, isle of Gotland) is of a more elongate obovate form, the anterior margin elevated, rounded, the terrace lines faint, the tubercles are larger than in the former Bumasti, oblong, curved, beanshaped, evenly rounded and smooth.

Another of the new species is one that LILJEVALL has found at Korpklint near Wisby (Pl. II figs. 41-46). The hypostoma is more elongate than transverse with the anterior margin regularly arcuated, by far not so prominent as in B. sulcatus and the former. By a shallow groove the exterior surface is divided in two fields, the anterior one the largest and covered with a few, distantiated, concentric terrace lines. The posterior field is smooth, nearly even and the maculæ lie just in the groove, with their axis oblique to the longitudinal axis of the hypostoma, beanshaped and smooth. The cephalic eyes show in a horizontal section hexaedral white walls enclosing a dark space in which some indistinct radii emanate, fig. 41 a. In the vertical sections the prisms are narrow and elongate and the integument by far not so thick as in Bumastus sulcatus, fig. 42, 43. The border zone of the eye in this (fig. 41) as well as in B. sulcatus, as seen in horizontal sections, agrees with that of the Asaphidæ.

=Bumastus= sp. inlet.

Pl. III fig. 1, 2.

In a detached piece of limestone, found in Gotland, at Norderstrand, near Wisby, two hypostomas and a free cheek lay embedded along with a coral of the genus Acantholithus and a Bronteus probably belonging to a new species. As the coral has not been found above the uppermost beds of the Lower Silurian it is probable that the trilobites also are derived from the same horizon.

The hypostoma, figured pl. III fig. 1, is broadly tongueshaped, with a short, blunt, triangular wing on each side of the anterior margin. The exterior side is nearly completely occupied by a single large field, which is convex and decorated by sparse terrace lines. Below this a smooth, nearly even plane, reaching to the inferior, elevated border of the hypostoma. The two tubercular macula' are situated exactly on the boundary line between the convex and the plane field. They are elongately ovate, pointed outwards, rounded inwards, with smooth and glossy surface. That portion, on which in the species of Bronteus the granular spot rests, is transversally wrinkled by some faintly elevated, sigmoid lines (fig. 2). It would have been of great interest to investigate the interior structure of these curious maculæ, but the scarcity of the material has prevented our doing so.

The other hypostoma, pl. III fig. 3-5, is larger and has the terrace lines more distantiated and forming more open curves, but for the rest it is of the same shape as the smaller, so that we may not ascribe these small deviations to a specific difference. The maculæ are more elongated, narrow, curved tubercles, thus differing from the next preceding. A section running through its left tubercle does not show any structure, but the shell of the maculæ is extremely thin, thus contrasting strongly with the surrounding thicker shell, fig. 4 a. The cephalic eye, again, remaining on the free cheek found in the same piece of limestone and probably belonging to the same specimen, has its surface well preserved and exhibits semiglobular lenses in low relief, fig. 5.

=Calymmene= BRONGN.

In the following species of this genus hypostomic tubercles have formerly been recorded.

Cal. Blumenbachi SALTER Monogr. pl. 8, fig. 9. The identity of the species is dubious.

Cal. duplicata SALTER Mon. pl. 9, fig. 22. The tubercles rather more linear and approximated than in the subsequent. SALTER calls them »two small transverse lobes ... forming a nearly continuous ridge».

Cal. tuberculata ANGEL. Tab. XIX fig. 5 c from the inside.

It is remarkable that the eyes or that part of them where the cornea and the lenses should be found, has been destroyed and entirely lost in all specimens of the Calymmene known to us, many hundreds having been searched and every one with a lacuna on that spot. It must have been of an excessive thinness. We know only of one instance, the Bohemian Calym. Arago, in which BARRANDE has seen the eye intact and provided with a small number of about eleven sphærical lenses.

=Calymmene intermedia= LDM.

Pl. III figs. 6, 7.

The clypeiform hypostoma is divided in two fields, one anterior larger, in the centre elevated in a short, blunt knob, and a posterior, bearing the two elevated rounded maculæ, just below the semicircular groove which separates it from the anterior field. These fields are surrounded by a large border which is posteriorly emarginated through a short rounded sinus. The surface is entirely covered by small semiglobular wartlets perforated by a straight pore or duct, which being filled with a black mineral, probably iron pyrites, gives a peculiar and characteristic aspect to the sections. In the same manner the whole mass of the hypostoma is pierced by straight, black lines, also filled up pores, starting from the outside and ending before attaining the inner surface.

The maculæ are prominent, separated from the hypostoma through narrow, distinct grooves, ovate, with the pointed end directed obliquely outwards, against the margins of the hypostoma. They are covered with wartlets and leaving only a little oblong spot free, quite smooth, this being consequently the macula proper. But in a longitudinal section there is not the least distinction between this macula and the other hypostoma, only, that it is free of pores.

=Calymmene tuberculata= BRÜNNICH.

Pl. III figs. 8, 9.