KONGL. sv. VET. AKADEMIENS HANDLINGAR. BAND 34. \:o 8. 9 



manner the granulated spol <>!' fehe hypostoma shows in a vertica] section even more 

 clearly (pl. II fig. 27) the elongate, coherenl lenses with a black centre and the 

 polyedric form «>l the lenses in a horizontal section (Pl. II fig. 26). The assumption of 

 a pair <>f small adventive eyes on the exteriör side of the hypostoma became thus based 

 on the clear evidencc of the perfect structura] agreemenl between them and the eyes oi 

 the head. But ii was aecessary to strengthen this evidence through extended pesearches 

 on the hypostoma of as many trilobite genera as possible and also to study the intimate 

 structure of the cephalic eyes in as many genera as could be accessible. 



We shall now give firsl a general account of the structure of the eyes in the tri- 

 lobites as far as we have been ;ible to study them, and then proceed t" describe the 

 maculse of the various genera, which we have observed. 



With regard to the prcsence or non existence of visual organs, we must remark 

 that ;i considerably greater number of genera than those which unanimously have been 

 regarded as blind, also are devoid of eyes though they still by many authors are ranked 

 amongst the oculate ones and we then had better first to make a review of them and 

 thus to eliminate them from the number of the oculate species. 



Blind Trilobites. 



In the chaos of generic forms and in the great disagreement which prevails as to 

 the systematizing of the trilobites of the Cambrian time, there is a thorough revision of 

 them highly needed by a person having access not only to the literature, but also to the 

 original specimens. It is almost impossible in the present state of things to tel l with 

 any degree of certainty how many well established genera had been living during that 

 period. Hence the difficulty of fixing the systematic names of many specimens the visual 

 organs of which are to be described. 



My researches on the visual organs of the Cambrian trilobites are founded on the 

 Angelinian Collection in the Swedish State Museum toirether with collections of foreiun 

 species, but also largely on the waste European and American literature, though we have 

 to deplore the often occuring inexactitude of the figures, especially in the older works, 

 and construeted or schematized figures in some of the newer ones, which give a quite false 

 notion of the structure. There is no lack of figures to show how it ought to be, accor- 

 ding to preconceived notions and, on the other hand, a great scarcity of representations, 

 to show how it reallv is. In spite of all this there is a sufficiently great number of 

 well established facts to demonstrate the organization of the Cambrian genera. 



The trilobites of this division may be called blind only in so far as they have no 

 eyes on the upper surface of the head, but they may have been provided with visual 

 organs, though more imperfect, on the hypostoma as really seems to have been the case 

 with some of them. 



