In the numerous specimens of which we have examined the hypostoma, there are always small, nearly circular maculæ which by their lighter colour are marked out from the surrounding smooth space of the inferior sinus of the lateral grooves where they are situated. They do not rise at all above the surrounding surface from which they are separated only by a fine, scarcely perceptible line. On the interior surface of the hypostoma they are better seen, and the enclosing line is deeper and more distinct. Their surface is there quite as smooth and even as on the exterior side of the hypostoma. They do not consequently in the least manner resemble muscular impressions as observable on the inside of the glabella of other trilobites. These are on the contrary elevated above the surface and finely striated in various ways.
The peculiar nature of these macula is revealed through the various sections we have made. In [pl. I fig. 15] a vertical section across the entire hypostoma is represented. The sectioned, dark maculæ (a) lie on both sides of the faintly curved central field, in the sinus of the lateral grooves, and the strongly developed terrace lines continue sidewards just a little on the interior side, where the duplicature turns round. As seen in a magnified vertical section ([fig. 16]) the maculæ consist of horizontal, whitish, straight lines, probably lines of successive growth, and these are crossed rectangularly by more irregular whitish lines separated from each other through dark spaces. The whole thus gains the aspect of a dark surface cancellated by white lines. This reticulated or spongious macula is enclosed as to its superior as well as to its inferior part in the compact and homogenous test of the hypostoma and occluded from the influence of the light. Its value as a visual organ consequently is insignificant. In a horizontal section ([fig. 17]) the structure is still more bewildering. There the whole macula is a confused, spongious white mass with dark spaces between the white meshes, and only at one side, the left one, some obscure indications as of polygones are visible. It would indeed have been impossible to interprete what this means, had not the study of the cephalic eye given a clue thereof.
The eye of this species is covered with a delicate perfectly smooth and glossy integument ([f. 12]) which is reposing immediately on the prismatic lenses of the cornea. In a few instances, depending on the colour or the state of preservation of that integument the lenses are indistinctly translucent. Owing to their state of preservation their aspect is greatly variable. They are in many instances, as seen in fig. 9, six-sided or some rhombic, foursided or even quadratic ([f. 8, 10]). They are all of the same length, 0,2 mm. along the surface proper of the eye, but are lengthened to 0,3 mm. towards the border of the eyes, [f. 12], which will be described further on; they are of equal breadth, amounting to 0,066 mm. Their inferior ends are slightly convex or nearly plane when well preserved, else, when as often is the case, disintegrated as to be scooped out and vaulted. Their interior structure, as revealed by sections, shows a cylindrical core, [f. 8-10], composed of concentric strata. This cylinder fills nearly the whole interior space of the prisms, there being, however, in many instances a compact dark mass between the cylinder and the walls of the prisms. There are also sections in which the prisms are filled with a uniformly black mass without any concentric structure. In longitudinal sections the outlines of the individual prisms are not clearly discernible ([f. 11]). There are longitudinal, white lines of varying thickness with lateral irregular offshoots, which may join with those from opposite walls, and give the interior a sort of spongious or cancellate appearance.
Towards both sides of the ocular surface, towards the superior and inferior side, a change sets in as to the shape of the lenses, as best seen in horizontal sections ([figs. 8, 10]) they are lengthened and become more and more indistinct, and at last in the upper and lower marginal zones pass over into a reticulate, spongious mass, which seen in a longitudinal section presents almost the same aspect as in the regular prismatic surface of the eye. [Fig. 11 b], [a] being the visual field. It is, however, more densely reticulate, but a prismatic arrangement is quite as much evident as in the ocular surface proper. In the rule the passage from the prismatic surface to the reticulate is gradual, the prisms becoming by and by irregular in their outline and diminishing in size ([fig. 12, a] the eye proper, [b] the border zone), but there are also instances where the distinction between these two fields is sharp and without any gradual transition. In the inferior reticulate zone there are generally some oblongue, funnelshaped pits. I am uncertain whether they are to be regarded as regular parts of the eye structure or rather as burrows of some parasite. They do not continue deep down.
If we now compare the reticulate zone of the eye with the maculæ of the hypostoma, for instance the vertical section [fig. 16] with [fig. 11 b], we find the most complete identity in structure. In the same manner the horizontal section of the border zone of the eye, [fig. 8 a], [fig. 10 b], and of the macula, [fig. 17] are similar. The same chaotic, spongious mass in both, with some tendency to form prisms more evident in the longitudinal sections, where the same reticulate structure with predominant white, longitudinal streaks is so palpable. That there is a complete identity in structure between the two, the macula of the hypostoma and the border zones of the eye, is as evident as anything can be, but as to the functional identity or what this function may have been it is difficult to decide anything with certainty, at least it seems to me that the capacity of vision must in both have been far more restricted than in the eye proper. They rather give the impression as of rudimentary visual organs.
Asaphus raniceps Dalm.
The maculæ of the hypostoma are placed obliquely in the saline manner as in the preceding species. They are more prominent and the oblong macula, with the longest diameter of 1,2 mm. is on the top of a little mound and surrounded by a fine, elevated marginal line ([fig. 23]). Its somewhat convex surface is entirely smooth and if sectioned horizontally exhibits the same sort of spongy texture as A. expansus ([pl. I f. 24]). In a vertical section ([fig. 25]) the macula does not occupy so large a space as in As. expansus, but rather lies as a lenticular disk in the hypostomic test closer to the superior surface than the inferior. With sufficiently high power the same sort of pillars, divided by horizontal strata is seen. Upon the whole the vertical section is not so clearly developed. The reticulate zone of the eye is more definitely separated front the prismatic zone than in A. expansus. There can be no doubt that there is a correspondence in this species between the structure of the macula! and the spongious zone of the eyes.
Asaphus cornigerus Schloth. (A. Kowalewskyi Lawrow).