72 LINDSTRÖM, VISUAL ORGANS OF THE TRILOBITES, 



The structure which characterizes the inacula as a visual organ, although often so 

 rudimentary, is not in all instances spread through the whole substance of the macula. 



This structure has in several species been restricted to a narrow circuinference of the 

 body of the macula, to its interiör apex as in Bronteus and Proetus. Only in the Asa- 

 phidae, in IllaBiius and Lichas the entire macula shows this structure. Perhaps, to judge 

 by certain indications in Bronteus, once in a larval or preceding stage of evolution the 

 whole surface of the macula was also in that genus covered with lenses, which have been 

 reduced. 



The different groups in which the examined genera raay be divided are the follow- 

 ing seven, showing the remarkably great diversity of these organs. There is even in the 

 same genus so great a variability that species with structure in the macula occur along 

 with those devoid of any structure or also, as in Lichas, with a different structure. It 

 must, however, be remembered, that the species of such genera may not be coeval. 



Group 1. In the sectioned maculse there is no trace whatever of any structure. 

 The test of the macula considerably thinner than that of the hypostoma. According to 

 the affinities of the genera they may be subdivided as follows. a Bumastus, Dysplanus. 

 b Nileus, Symphysurus, ? Ogygia. c Calymmene, Homalonotus. d Chirurus pro parte. 



Group 2. The whole inacula is of a spongious or irregularly polyedric structure, 

 showing prisms in vertical sections. Its concordance with the structure of the deteriorated 

 cephalic eyes or with the so called börder zone is complete. If a supposition may be 

 hazarded, I think that the spongious or reticulate structure in the maculaj is their real 

 and original state, a lower stage of development of the visual organs, out of which the 

 prisms in the cephalic eyes have been formed, and that the prismatic lenses in their de- 

 cay, as seen in the Asaphidse, reveal to us their original state and structure, and thus, 

 as it were, return to the primary stage in the maculae. 



Asaphus, Isotelus, Megalaspis, Ptychopyge, Niobe, ? Megalaspides, ? Barrandia. 



Illaenus, Lichas, Trochurus, ? Platymetopus. 



Encrinurus also probably belongs to this group. 



Group 3. Maculae with well developed globular lenses on the interiör third of the 

 macula. The blank part of the macula without any structure. In one instance (Chirurus 

 spinulosus) the whole macula covered with lenses. 



Bronteus, Chirurus spinulosus. 



Group 4. The macube form sunk pits with smooth bottom. Structure unknown. 

 In a few species there seems to be no less than four maculae in two pair above each other. 



Dalmanites. 



Group 6. The elongate straight macula' carrying on their innermost point from 

 three to tive diminutive, segregate ocelli situated on a clear, white patch. Proetus. 



As yet these ocelli have not been observed on the related genera Cyphaspis, Phaé- 

 tonides, Phillipsia and Griltithides, but they may be preliminarly ranked here in con- 

 sequence of their close conformity. 



Group 7. The maculse have in this group been reduced to a pair of tiny ocelli 

 situated high up on the hypostoma, near its anterior margin. 



Phacops, Aeaste. 





