KONGL. SV. VET. AKADEMIENS HANDLINGAR. BAND 34. \:m 8. 25 



a. un$egmcnte<l, Precambrian, adult form unknown, supposed to be like the firsl 

 larval stage of Liostracus aceording to the figure I of Brögger and also as the head of 

 Agnostus parvifrons Linnarsson. Corresponding larva] stages Liostracus (I). 



b. segmentet!. Probably precambrian, not represented as adult in the Cambrian 

 strata. Supposed to be like the Larval forms 11 and III of Brögger's Liostracus, con- 

 sisting of the round or oval head shield and the segmented glabella. Also Barrandes 

 first stage of Sao (Barr. fig. 1 pl. 7). 



As a transition to the next phase forms may be tmagined having head and the 

 beginning pygidium, nearly as Agnostus atavus, minus the thorax. The corresponding 

 larval stages are Liostracus (Brögger's figures IV, V, VI) and Sao (Barrande, pl. 7 fig. 

 2 — 4 a, b). Nearly so, though the pygidium is more developed, are the larva of Agn. 

 bibullatus and Agn. nudus (Bark, pl. 49) both without thorax. 



II. 



These have the three chief parts of the body developed. 



3. The facial ri</g< . Cambrian, adult with glabella and facial ridges, short or long, 

 emanating from the top of the glabella, thorax and pygidium. Several American Cono- 

 coryphaj. Corresponding larval stage, Sao, stage 3, Barr. pl. 7 figs 4 d — 9. 



4. Suture. A suture dividing the fixed cheeks in two pair viz. two fixed cheeks 

 and two free cheeks. Fully developed facial ridge. Oldest known adult forms are Ellipso- 

 cephalus, Arionellus, already in the Lowest Cambrian, the Olenellus beds, what presupposes 

 a long, antecedent lineage far back in the precambrian times. Corresponding larval stage 

 in Sao, Barr. pl. 7 figs 10 — 13. The plurality of Cambrian trilobites belong here. 



An intermediate stage leading to the next is seen in such forms in which the 

 ridge posteriorly is widened into the »eye lobe», which rests in the scallop of the free 

 cheek. So it is in Liostracus and many others besides. Solenopleura possibly oculate. 



III. 



5. Globular eyes. Cambrian, in the youngest zone, the Olenus schists. The oldest 

 at present known oculate trilobite Eurycare is found in the second division of these schists. 



In the lowest Lower Silurian division, the Ceratopyge limestone, Euloma and Cera- 

 topyge occur as the last survivors of the blind, partially ridge bearing genera. The 

 Trinucleidae and Ampyx belong to another group of trilobites. 



Even among other exclusively Lower Silurian genera, in which the plurality of the 

 species is oculate, there are species entirely blind. So with Illsenus, in which genus Dr 

 Holm has not found any eye in 111. Angclini, I. leptopleura and 111. csecus. The free 

 cheek in these three species is much narrow, as the facial suture lies near the margin 

 of the head. 



