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 



o 



that the eyes of Trilobites resemble those of Isopods, 1 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 Triarthrus, Euloma, 

 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 Trinucleus eyes are present, 



but have been stated to 

 be of a different type, 

 Thev 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 ; 



FIG. 140 Trinucleidae. A, Oronnh./ms elatifrons, it appears to Consist ot 

 Ang. < 5. Restoration based on specimens twQ Qr three granules 01* 

 from the Upper Cambrian (Tremauoc) of 



shim-ton, Shropshire. B, Tri,i<i,-i>-iis buck- tubercles wliich are really 



I,,*,!! Burr Ordovician. Bohemia A com- j d connected 



plete but not fully-grown individual showing 



eyes. Natural size. (After Barrande. ) C, with the front of the 

 Ampyx rouaulti, Barr. x 3. Ordoviciau, i i 11 i ., pvp i; np 

 Bohemia. (After Barrande.) 



No facial suture can be 



seen, consequently the whole of the cheek is stated to be the 

 fixed cheek.' 2 In Trinucleus (Fig. 140, B) a single tubercle is 



1 Watase, Johns Hoplcins Univ. Studies, Biol. Lab. iv., 1890, p. 290. Lindstrom, 

 op. cif. p. 27. 



2 A suture is said to be present at the external margin of the flattened cephalic 

 border. 



