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.
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.