10 



LINDSTRÖM, VISUAL ORGANS OF THE TK1LOBITES. 



According to the structure of the head shield the l)lind trilobites may be subdivided 

 into the following well characterized groups. These are: 



I. Without facial ridge: 



1) The Archaean Trilobites. 



II. With facial ridge: 



1) The Oleuellidae. 



2) The Olenidae and related. 



I. Blind trilobites without facial ridge (= »eye lobe»). 



Group 1. The Archaean Trilobites. 



In these the head shield is in one piece without anv facial suture and facial ridge, 

 and without the least trace of anything that niight be called a visual organ and they 

 must consequently be considered as totally blind. In contradistinction to the following 

 groups, excepting the oldest Olenellidae, the head consists of only three parts, 1 ) glabella, 

 and 2 — 3) the two fixed cheeks. These genera range from the oldest zone in which 

 hitherto trilobites ha ve been found, that of Olenellus (Holmia) Kjerulfi, to the zone of 

 Paradoxides Forchhammeri, and sorae, as Agnostus, even continue as high in the Lower 

 Silurian series as in the Brachiopod schists. Beside Agnostus the other genera are Cono- 

 coryphe (seven species in the Swedish Cambrium), Toxotis, Ctenocephalus (?), Elvx, Aneu- 

 acarithus, Conophrys and Microdiscus. 



»Harpides» breviceps Ang., also belongs here. Anopocare of Angelin should also be 

 regarded as one of this group. But it cannot be retained anv longer because it is founded 

 on two other, well known forms, being, according to Linnarsson, Peltura scaraba?oides 

 (pl. 27 iig. 1, a in Pal. Scand.) and young specimens of Splnerophthalmus alatus (ibid. 

 tigs. 1 & 2). 



It is remarkable that some of the Conocoryphida' have an imperfect facial ridge, 

 to be compared with the comnienced one in Sao Bakh. I (pl. 7 Hg. 9). »So the 

 American (Jon. trilineata and reticulata Walcott U. S. Geol. Survey lOth Rep. pt. I, 

 pl. XCV f. 5 & ti. It is, as it were, arrested in the development and these adult trilo- 

 bites had stopped, where the larva of Sao was proceeding in its second stage. They are 

 the forerunners of the blind trilobites with facial suture, belonging to the third group. 



It needs scarcely be nientioned that the genera now enumerated have hardlv any- 

 thing in common, beside the general character of the head, and that real affinity exists 

 otdy between Agnostus and Microdiscus, and probably also between Conocoryphe, Oteuo- 

 cephalus and Elyx. 



Beecheh 1 asserts that there is a suture in Agnostus, but in vain wc have 

 searched for it in numerous well preserved specimens and Dr Holm also denies its pre- 



Nat. Classificatioii of the Trilobites, p. 183. 



