50 



LINDSTKO.M, VISUAL ORGANS OF THE THILOBITES. 



front sidc and there is a posterior lateral wing, midways between the anterior and pos- 

 terior margin short, blunt, directed obliquely backwards. The whole exteriör surface is 

 covered with diminutive tubercles, the above mentioned smooth spöts interspersed 011 the 

 central disk. A little below the niiddle there are two lengthened pits, so shallow that 

 they often are not discernible. In a specimen of Chirurus spinulosus Nieczkowski from 

 Estland there are two, almost 3 mm. long döts, one on each side above the maeulae as 

 the fig. 21 shows and in several others of this generic group there are also indications 

 of similar. I cannot comj)are these döts with anything morc than the lengthened spöts 

 visible in Acidaspis crenata. 



The macuhe are wanting in this species, but the cephalic eyes are well developcd. 

 A section of theni tig. 19 shows the enormous difference of the shell in the eye and 

 the surroundings where the shell surpasses it many times in thiekness. The eye consists 

 in fäet only of the ovate, beadlike lenses of which a string is seen sectioned in the fignre 

 mentioned. Morc enlarged (fig. 20) a nucleus is visible in each lens, and in a hori- 

 zontal section (tig. IS) a littl< j below the surface, where they are more pressed against 

 themselves, they have a polygonal shape. The fignre 17 seenis to represcnt lenses that 

 have been much changed interiorly, having only a narrow zone left of the primary structure. 



Chirurus glaber Angelin. 



Pl. III tig. 11. 



As a sainple of the shape of the surface of the eyes in "this genus, we have given 

 ;i ftgure, showing the small rounded lenses. 



Chirurus ornatus Dalman. 



Pl. III tigs. 12—14. 



The cephalic eyes Inve glojbular lenses, of more than double the size of those in 

 Chir. clavifrons. The difference between the lenses and the surrounding shell amounts 

 at the highest to thrice the former, while in Ch. clavifrons it is at least seven times as 

 much. In a horizontal section the lenses have a little, darkish irregular nucleus sur- 

 rounded b}" a radiated structure (hg. 12). 



The maculaa (fig. 14) are slightly concave, oblong, smooth, surrounded by compara- 

 tively large granulations. 



Chirurus speciosus Dalm. 



Pl. IV tig. i. 



The exteriör surface of the hypostoma is sparingly covered by granules <>1 larger 

 si/c than in other Chiruri. The maculte as scen in ;i cast are prominent near the pos- 

 terior börder, sunk in their centre and surrounded by a narrow distinct börder line. In 

 a horizontal section it has not been possible to detect any peculiar structure, <>nly a dark 

 börder surrounding the interiör clear surface. 



