TRILOBITA. 



363 



border, and the postero-lateral angles are often produced into 

 long backward-directed processes. 



The pleural regions of the head are known as the genae or 

 cheeks, and bear the large reniform, many-facetted eyes 

 which are in some cases raised on prominences of the head 

 shield. In Harpes the eyes are replaced by groups of two or 

 three ocelli, and in some genera they were absent altogether. 

 The gena of either side is traversed by the facial or ocular 

 suture, which runs forwards 

 from the hinder or outer mar- 

 gin, passing on the median side 

 of the eye, and is continuous 

 with its fellow in the middle 

 line in front of the glabella. 



From the fact that in decayed 

 specimens the regions of the head 

 fall apart at this suture it is con- 

 jectured that some small amount of 

 movement between the head regions 

 may have "been admitted during life 

 and the terms " fixed " and " mov- 

 able cheeks " have been applied to 

 the areas of the genae internal and 

 external to the ocular suture. 



In addition to the eyes on the 

 upper surface of the head, some trilo- 

 bites possess structures on the under 

 surface which are also, apparently, 

 eyes. They are situated on either 

 side of the posterior third of the 

 hypostome. Each consists of a 

 small oval area, called a macula by 

 Lindstrom. In some genera they are 

 smooth ; in others they are partly or 

 entirely facetted, and resemble in FIG. 

 structure the dorsal eyes.* 



The free trunk-segments vary in number from two (Agnostus) 

 to twenty-nine. Those of the pygidium also vary in different 

 genera (from 2-28, though the number is by no means com- 

 plementary to that of the free segments), as does the complete- 

 ness of their fusion into a uniform plate. 



The terminal segment (telson) is without appendages and the 

 anus is situated on its under surface. The sternal region of 



* G. Lindstrom, Researches on the Visual Organs of Trilobites, Kongl. 

 Svenska Vetenskaps Akad. Bandet 34, 1901. 



244. Dorsal surface of Triarthrus 

 Becki. After Beecher. 



