KONGL. SV. VET. AKADEMIENS HANDLINGAR. BAND 34. \:<> 8. 21 



(of the eye) est toujours mal conservée, pour nous permettre de voir si olle étail réti- 

 culée.» And he also confounds the ridge, filefci as he names it, with the eye itself, and 

 the first faint beginnings of this ridge in his fourth stage he considers as the eye. 1 I>\ 

 a partly schematical figure of the free cheek (fig. 2")) ho places the eye on lins cheek, 

 and in the same manner in tig. 29, »restaurée d'aprés divors fragments», he figures a 

 reticnlated surface of the eye on the free eheek, ontside the tuberele. I have songht for 

 a reticnlated surface on suffieiently good specimens, but never found any, and I mn-t 

 consider Sao as one of the blind genera. Barkande himself also in the table on the 

 eyes of the trilobites places Sao in the group with »Surface visuelle inconnue», p. 131. 

 Brögger has also succeeded 2 in finding a series of small larva, which he considers 

 as belonging to a species of Liostracus. As the figures drawn by Brögger twenty five years 

 ago may now be verv little known, I here reproduce them with the kind permission of 

 the Editors of »Geologiska Föreningens Förhandlingar», where they were published in 1875. 



u § © © 4D <G) 



I II III IV V VI VII VIII IX 



(O is new, added here to supplement fig. I.) 



This development proceeds nearly upon the same plan as in Sao. The first stage, however, 

 (I) seems to be much earlier than any of Sao, the rhachis or glabella consisting of an unseg- 

 mented ridge of more primitive appearance. Before this first stage of Brögger\s larva a still 

 older phase of development can be imagined, (O) a simple ronnded, smooth head shield 

 without any indication of a glabella at all. This stage might correspond to the head of 

 certain species of Agnostus, as A. glandiformis, A. nudus, which have no glabella. This 

 stage O is also valid for the larva of Sao. Several stages are evidently wanting between I 

 and II, in the låter of which the thickened glabella is divided in four segments. In III we 

 have six segments, all these three stages (I — III) consisting only of the ovate head with 

 narrow fixed cheeks. In IV the pygidinm has been added to the primitive head, but the 

 segments of the glabella have been reduced to four and in V slightly altered in shape 

 In VI, again, we see the head with five glabellar segments and scarcely the first sign of 

 the facial ridge. Between VI and VII there must be links missing, as the change can not 



o o" o 



be so abrupt, and likewise between VIII and IX as in VIII there are still no free cheeks 

 nor any facial ridge. This interesting discovery of Brögger confirms, together with those of 

 several other authors 3 , the supposition that the development of the låter Cambrian and 

 older Silurian forms is a qnite different one from that of the Olenellidas and the Para- 

 doxida?. They have a rhachis, but no pleura proper, as the single facial ridge has a 



i p. 389. 



2 Fossiler fra Öxna og Klettna, Geol. För. Förhandl. 1875, p. 572, pl. 25, fig, I — X. 



3 Foremost among these stauds Matthew in »Illustrations of the Fauna of St. John» IV, where he, p. 

 143, pl. II figs 1 f. etc, describes a few stages verv like those given by Brögger, so the glabella as an un- 

 segmented, narrow ridge etc. 



K. Sv. Vet. Akad. Handl. Baud 34. N:o 8. 3 



