40 LINDSTRÖM, VISUAL ORGANS OF THE TRILOBITES. 



they are of equal breadth, amounting to O.oöb mm. Their inferior ends are slightly con- 

 vex or nearly plane when well preserved, else, when as often is the case, disintegrated as 



to be scooped out and vaulted. Their interiör structure, as revealed by sections, shows 

 a cylindrical core, f. 8 — 10, coniposed of concentrie strata. This cylinder tills nearly the 

 whole interiör space of the prisins, there being, however, in many instances a compact 

 dark mäss between the cylinder and the walls of the prisma. There are also sections in 

 which the prisms are filled with a uniformly black mäss without any concentrie structure. 

 In longitudinal sections the outlines of the individnal prisins are not elearly 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 interiör 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 (tigs. 8, 10) 

 they are lengthened and become more and more indistinct, and at last in the upper and 

 lower marginal zones pass över into a reticulate, spongious mäss, which seen in a longi- 

 tudinal 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 (tig. 12, a the eye 

 proper, b the börder 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 sotne oblongue, funnelshaped pitts. I am uncertain whether they 

 are to be regarded as regular parts of the eye structure or rather as burrows of sonie 

 parasite. They do not continue deep down. 



If we now compare the reticulate zone of the eye with the maculae of the hypo- 

 stoma, for instance the vertical section fig. 16 with fig. 11 b, we find the raost complete 

 identity in structure. In the same nianner the horizontal section of the börder zone of 

 the e}'e, fig. 8 a, fig. 10 //, and of the macula, fig. 17 are similar. The same chaotic, 

 spongious mäss in both, with somc tendency to form prisms more evident in the longi- 

 tudinal 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 börder 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 diffi- 

 cult 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. 



IM. I t'. 23— 2G. 



The macula: of the hypostoma are; placed obliquely in the same manner as in the 

 preceding species. They are more prominent and the oblong macula, with the longest dia- 



