230 



TRILOBITA 



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 Triarthrus, Euloma, 

 and some species of Calymene from the Ordovician ; in Arethusina 

 and Acidaspis from the Silurian ; and in Harjyes 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. 

 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 ; 



Fig. 140— Trinucleidac. A, Oromefopus elatifroiis, it appears to COnsist of 



Ang. X 5. Restoration based on specimens ^^^^ ^^ ^j^^.^^ granules Or 



from the Upper Cambrian (Tremadoc) of _ o 



Shineton, Shropshire. B, Trinudeus buck- tubercleS which are really 



landi, Barr. Ordovician, Bohemia. A com- j^^^gg^ ^^^^ -^ connected 



plete but not fully-grown individual showing 

 eyes. Natural size. (After Barrande. ) C, 

 Avipyx rouanlti, Barr. x 3. 

 Bohemia. (After Barrande.) 



with the front of the 



Ordovician, i i n i t 



' glabella by an eye -line. 

 No facial suture can be 

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

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



^ Watase, Johns Hopkins Univ. Studies, Biol. Zab.iv., 1890, p. 290. Lindstroin, 

 op. cit. p. 27. 



^ A suture is said to be jn-esent at the external margin of the flattened cephalic 

 border. 



