Part 13

D. rugosus BARR. pl. 24 f. 23. The two maculæ in the ordinary place, without being united by a groove.

D. socialis BARR. pl. 26 f. 21. Rather of an uncommon shape, not so triangular as in the other species, the two maculæ distinct as narrow slits. P. 553 »Vers l'extrémité du corps central, on aperçoit de chaque côté, près du bord, une impression oblique, alongée et arquée».

D. spinifer BARR. Pl. 25, fig. 20. A large beautifully preserved hypostoma, shows what I think we positively must interpret as a double pair of maculæ close above each other. As stated above there is reason to believe that in other species of Dalmanites there also are indications of four maculæ, though not so evident as in this species. BARRANDE says nothing about this remarkable feature.

=Dalmanites imbricatulus= ANGELIN.

Pl. III figs. 43, 44.

The interesting image, as given by the agglomerated eyes, is represented in the magnified figure 43 Plate III. The lenses lie there separated from each other at much varying distance, some in close contact. The presence of the delicate covering membrane is clearly seen, being of white colour contrasting with the black, glossy lens. It is much lacerated and only preserved around the periphery. The size of the ocelli is on an average 0,5 mm., a little one has 0,3 mm. The surface between the ocelli is most finely granulated. The granules scarcely attaining the fourth of the granules in Dalm. vulgaris.

In a vertical section the regular biconvex lenses are seen to be covered with the extremely thin integument, which is a direct continuation from the test. The test between the lenses is perforated by some longitudinal tubes, as usual in the skeleton of the trilobites. We have not found any hypostoma with the maculæ.

=Dalmanites obtusus= LDM.

Pl. III figs. 45, 46.

We have not succeeded in finding any hypostoma, but as the structure of the cephalic eyes is sufficiently well preserved, we here describe it. The lenses seen from the surface look globular, and in a vertical section they are ovate and in a horizontal circular. They are covered by a very thin and delicate membrane, that envelops their superior moiety completely and between the eyes it is enclosed by the surrounding test and growing out from it. In the horizontal section, fig. 45, it looks as a circular frame around the lenses and the membranes of two contiguous lenses sometimes lie so close, that they entirely fill up the interspace between the lenses, leaving only a narrow slit between them marking their boundaries. The space between the lenses, fig. 46, is porous, being perforated by tubes, which continue vertically down through the shell of the cheek.

Dalm. sclerops DALMAN.

Pl. IV fig. 3.

Commonly this species has been ranged with Phacops, but it is evident that it as to the conformation of its glabella and the head in general is highly discrepant, and in these respects is more concordant with Dalmanites, though still deviating as for instance in the pygidium and some details of the head. Therefore it may be justified with FRIEDR. SCHMIDT to consider it as a generic division, though not, as he has it, as an independent genus, but as a subgenus of Dalmanites.

The granulated hypostoma differs from that of Dalmanites proper in having the anterior margin strongly arched, the pointed wings almost on the median line of the hypostoma. There is one groove near the posterior border and the maculæ lie as two, narrow long, crescentic grooves close below the wings. The few sections give no clear idea of any structure.

The cephalic eyes are constructed on the same plan as in the previous species, the lenses being surrounded by a frame.

Dalm. vulgaris SALTER.

Pl. III figs. 47-52.

The hypostoma has the common shield-like shape, fig. 51, anteriorly slightly arcuated, the short obtuse wings curvated towards the interior surface. It is finely granulated. The groove does not go so near the cuspidate posterior margin as in other species. The two maculæ, fig. 52, are narrow and elliptic, somewhat convex and lie in a little concavity, a narrow smooth space around is devoid of the granules, which cover the surface of the rest. They show absolutely no structure, excepting a few indistinct blackish spots.

The cephalic eyes have been often described and delineated. Most detailed are the descriptions and figures given by SALTER in the Memoirs of the Geological Survey Dec. II. pl. 1. In the figure 4 a part of the surface is shown in well preserved state. He says that »the cornea is ... present and distinctly convex over each lens, the intermediate portions being ornamented with tubercles and granules». He thinks also that there are undeveloped or small lenses between the larger. His fig. 5 represents the frames around the lenses, these being probably lost. His explanation of fig. 6 I cannot understand. There are weathered or spoilt lenses figured.

There is no doubt that the granulated surface of the head continues between the more or less free lying lenses as shown here in a new figure taken from a specimen from Dudley, fig. 47. The granules of the surface are larger and more rare between the ocelli than for instance in Dalm. imbricatulus. Small lenses, undeveloped as SALTER calls them, may in fact only be such granules of somewhat larger size than usual. Lenses having the appearance as in SALTER'S fig. 6, as if a covering was partially destroyed and the lens visible below it, I have also found in Swedish specimens, as represented in fig. 47. In a vertical section, fig. 50, a delicate covering integument is seen and the oval lenses lie regularly with their frames around them quite as in Dalmanites obtusus and in horizontal sections, figs. 48, 49, there are also the same sort of black, irregular dots, being the sectioned tubes of the surface between the lenses. But in another feature there is great interest. Beneath each lens there are fascicules of tiny rods, twice as long as the lenses. They are represented in a vertical section in fig. 50 and in a tangential section in fig. 49.

As our imperfect knowledge of so delicate anatomical structures in fossil crustaceans does not admit of secure comparisons with the visual organs of recent crustaceans, no suggestion can be given to interpret their nature. Probably they have no connection with the structure of the eyes and it is not even certain that they are of organic origin.

=Dysplanus= BURM.

If there were no other characters to distinguish this genus from Illænus its strangely deviating hypostoma must do it. We have examined two species and in both the hypostoma is almost oval, evenly rounded both anteriorly and posteriorly. Near the anterior border two narrow, long hornlike wings project. They are flattened, thin and lamellar from the basis and end in a hollow, acuminated point.

=D. centrotus= DALM.

Pl. III figs. 53-56.

This species has the maculæ placed near the posterior border, in a semilunar flat, devoid of the terrace-lines. The maculæ are elongated tubercles and do not show any structure when sectioned. The cephalic eyes have a thin membrane covering the short cupshaped lenses, which, closely packed, in the transverse section give the usual polygonal mosaic of hexaeders. They have a radiate structure, when decayed.

D. ladogensis HOLM.

Pl. III figs. 57, 58.