230 TRILOBITA CHAP. 
alternating rows. In Trilobites with a conical eye the outer 
segment of the cone bears the visual surface. It has been stated 
that the eyes of Trilobites resemble those of Isopods,’ but close 
comparison is difficult to make, since in Trilobites no part of the 
eye beneath the lenses is preserved. In some genera a thread- 
like ridge, called the “eye-line,’ passes from the glabella, 
generally from the front segment, to the eye, where it often ends 
in the palpebral lobe; this eye-line is found in nearly all 
genera which are confined to the Cambrian period, and persists 
in a few of later date, as for example in Zriarthrus, Huloma, 
and some species of Calymene from the Ordovician ; in Arethusina 
and Acidaspis from the Silurian; and in Harpes from the 
Devonian (Fig. 150, A). 
In Harpes and in some species of 7rinucleus eyes are present, 
but have been stated to 
be of a different type. 
They are described as 
simple eyes, and have 
been compared with ocelli ; 
they are never found in 
Trilobites which possess 
the compound eyes de- 
scribed above. In Harpes 
(Fig. 150, A) the eye is 
near the middle of the 
cheek, in the position 
where compound — eyes 
occur in other genera ; 
Fic. 140—Trinucleidae. A, Orometopus elatifrons, it appears to consist of 
dog 5S fawn based oh see tayo, on, three @ringleaon 
Shineton, Shropshire. 13. Trinucleus buck- tubercles which are really 
lend, Beer Ordovician, Bohemia. A com: Jenses, and is connected 
eyes. Natural size. (After Barrande.) C, with the front of the 
Ampye round, Bar. > 3 Ortorea®, stabella by an eye-line 
No facial suture can be 
seen, consequently the whole of the cheek is stated to be the 
fixed cheek.” In Zrinucleus (Fig. 140, B) a single tubercle is 
1 Watase, Johns Hopkins Univ. Studies, Biol. Lab. iv., 1890, p. 290. Lindstrém, 
Op» Clipe aie 
2 A suture is said to be present at the external margin of the flattened cephalic 
border. 
