44 LINDSTRÖM, VISUAL ORGANS OF THE TRILOBITES. 



are situated above the groove near its superior sinuses. They are elongated, fig. 17, 

 elliptic with the uarrow pointed end directed outwards and the broad rounded end 

 inwards. The chief surface is seooped out as a shallow depression. The granulated spöt 

 is situated on the broad er end and covering it completely. The relatively large lenses 

 are arranged in five regular rows, the uppermost one being the longest. On the interiör 

 surface of the hypostoma there are the corresponding sockets of both maculae with 

 smooth surface. The horizontal sections of the granules tigs. 18, Ii) present the image 

 of white rings in close contact, without, however, to occasion a prisinatic structure, a 

 dark interspace lying between each ring. These lenses are filled with a dark mäss, and 

 in some the same sort of radiated structure is perceptible as in the lenses of the ce- 

 phalic eye. In horizontal sections of the cephalic eye the lenses approach the polyedral 

 shape. In another section near to the surface of another specimen the lenses are de- 

 cidedly hexaedr.nl. The vertical sections, fig. 14, reveal their real nature as lenses where 

 they lie as a string of beads with a dark nucleus enclosed within a thin whitish shell. 

 They are covered by a thin membranous lining. When seen in transmitted light the 

 lenses proper are dark, and the shell white and in refiected light the lenses are lighter 

 th.iti the rest. 



Br. laticauda Ängel. 



Pl. II fi£s. 6 — 13. 



Lower Silurian from Dalecarlia. The hypostoma, f. 10, is of a broad clypeate shape, 

 the anterior margin rounded without any large projecting wings. On the exteriör surface 

 there are two grooves parallel with the rounded interiör margin. As in Br. polyactin the 

 two elliptic tubercles are situated on the inferior edge or slope of the upper groove and 

 they are deepened by ;i shallow depression as in Br. platyactin. The granulated spöt 

 situated along the posterior margin of the macula deviates much in shape from that 

 of the other species. It consists of a long and narrow stripc ending in a fine point out- 

 wards and widening inwards, where it is rounded, forming thus a claviform, curved ele- 

 vation. Around the granulated area the macula? are quite smooth, and show in a hori- 

 zontal section an irregular structure of tiny black döts nearly resembling the structure 

 of the hypostomic shell. We have not been able to obtain any good vertical section, hut 

 by casts of the interiör side of the tubercles it is found that the granules form polyedric 

 facets like those of the eyes though perhaps not so regular. There is a specimen with 

 the granules intact, and the polyedric shape is then not so distinct. 



Sections of the eyes elucidate the structure, which partially is obscure in other spe- 

 cies of Bronteus. There is a thin membranous coating covering the subjacent well formed 

 lenses (fig. 8). When this membrane in some instances has been peeled off the lenses lie 

 bare (fig. 9). These have a dark nucleus and in some instances it seems as if there were 

 two. In a vertical section they look sphcroidal, in a horizontal section again they are 

 polyedric, especially when taken somewhat below the surface or near the middle Ii ne 

 (fig. (!). In fig. 7, some lenses are delineated in a horizontal section, highly magnified 

 and the corallian appearance is evident. The lenses are well separated by distinct lines, 

 and from their inner tubes whitish reticulations radiatc towards the centre. 



