24 LINDSTRÖM, VISUAL ORGANS OF THE TR1LOBITES. 



We have from the lowest Olenellus beds to the lowest Lower Silurian strata fol- 

 lowed the clear traces of the changes from the eyeless Olenellidaa past the Paradoxida 1 

 with the facial suture to the great tribe of the Olenidte in which the facial ridffe with 

 its lobe is so prominent, and to the oculate genera at the top of the formation. 



As stated above there seems to be evidence enough for aecepting two different lines 

 of evolution in these the oldest trilobites. For the oldest, Olenellus is the type and for 

 the second Sao may be taken as the representative. 



These two evolutional lines could be represented with their phases in chronological 

 succession as follows: 



A. The Olenellus line. 



1. The trilobite consisting only of the head with rhachis and pleurse, no sutures, 

 no ridge, no eyes. Adult forms precambrian. Corresponding larval forms (Olenellus 

 asaphoides) in the Lowest Cambrian. Still older and simpler forms may be presupposed 

 as preceding these. 



2. Semilunate facial ridge on the tripartite head-shield, no sutures, no eyes, but 

 a hypostoma fused to the rostrum of the head and provided with macula?. The adult 

 animal with thorax and pygidium in the oldest Cambrium, the Olenellus zone. Genera: 

 Olenellus, Holmia (with beginning suture), Mesonacis, Schmidtia. 



3. The head quinquepartite with facial suture, short semilunate ridge in the adult, 

 a long ridge in the young, no eyes. The genera occur in the Paradoxides-zones of 

 the Cambrian formation. These genera are Paradoxides, Centropleura, Metadoxides, Hy- 

 drocephalus, which are direct descendants of the Olenellida?. 



This line of evolution, in which the species never acquired eyes, was extinct in 

 the sixth zone of the Swedish Cambrian, Centropleura Lovéni being the last. 



B. The Sao line. 



I call this so because we see in the development of Sao, as represented by Bar- 

 rande, most of the different phases in evolution, which the ridge bearing trilobites of 

 this second group have experienced during the Cambrian times. 



I. 



The body consists only of the head, neither pygidium, nor thorax yet developed. 



1. Psilocephali (ipikos, bald), primeval, precambrian, adult stage not found, supposed 

 to be like the head shield of Agnostus glandiformis and A. biplicatus quite round and bare 

 without any glabella nor sutures. The corresponding stage in development also wanted. 



2. Glabellate. The entire animal only head with a mesial ridge, which at first 

 is entire or unsegmented and låter is metamorphosed into the seginented glabella. 



