22 LINDSTRÖM, VISUAL ORGANS OF THE TRILOBITES. 



quite different signification and appears at a comparatively much låter stage than the 

 facial ridge of the Olenellidip, which is present from the earliest stages known. 1 



This form of facial ridge, although prevalent in the middle and låter Cambrian 

 times, dates back so far that genera coeval witli Holmia, viz. Ellipsocephalus and Ario- 

 nellus show it along the facial suture. This early occurence of the ridge coeval with 

 the less developed Olenellidae leads to the assumption of a different origin of these trilo- 

 bites as a branch, which already far back in the oldest Cambrian or precambrian times 

 had deviated from a common ancestor and I have tried to give a view in tables of these 

 two coordinated lines of evolution further on, at pages 24 and 25. The most remarkable 

 genera which, as far as I have found, belong to this fourth group are Ellipsocephalus, 

 Arionellus, Liostracus, Olenus, Leptoplastus, Parabolina, Corynexochus, Parabolinella, Sao, 

 Ptychoparia, Doropyge, Oryctocephalus etc. 



I have not been able with an absolute degree of certainty to recognize whether 

 some of these genera now enumerated, have been oculate or blind like Sao, like the Pa- 

 radoxida? and similar. The precarious state of preservation prevents all definite conclu- 

 sions in that respect. It seems, however, that the evidence gathered through the exami- 

 nation of numerous specinicns rather points in a negative direction. As the free cheeks 

 in these old Cambrian trilobites have been in a very loose connection with the fixed 

 cheeks and generally deciduous, contrary to the condition in the Silurian ones, it is in 

 many instances very difficult to tell whether a species with facial ridge, especially those 

 from the earlier Olenus schists have been blind or provided with eyes. 



The order of succession of the genera in the Swedish uppermost Cambrian, in the 

 Olenid slates is, according to S. A. Tullberg's researches 2 at Andrarum in Scania as 

 follows : 



1. Parabolina (oldest division of the Olenus slates). 

 Olenus. 



Liostracus. 



2. Eurycare. 

 Leptoplastus. 



3. Pel tura. 

 Spha?rophthalmus. 

 Ctenopyge. 



4. Cyclognathus (uppermost). 

 Acerocare. 



It is already in the second division that we find the earliest oculate genus in Eu- 

 rycare and in the higher strata Sphaerophthalmus and Ctenopyge with enormous hemisphae- 

 rical eye balls, while Olenus and Parabolina, their earliest predecessors, probably were 



1 A deviating form of the ridges is shown hy »Liostracus» tener Hartt, Acad. Geol. 2d Ed. p. 652 (see 

 alf > MATTHBW Illustrations of the Fauna of St. John [1887] p. 132 p. 1 f. 3 a — 3 c). Beside the usual 

 facial ridge there is a second pair of ridges between the tirst and the glabella, arching in an opposite direction. 

 See also WALCOTT, Bulletin U. 8. Geol. Survey, N:o 10, 1884 pl. V, figs 6, 6 a, G h, new ligures and copy 

 of I1ahtt's description. 



2 Om Agnostusarterna vid Andrarum. 



