196 PALAEONTOLOGY 



apodemes serve principally for the attachment of the 

 limb-muscles, this seems to indicate that in Calymene the 

 head-limbs are more specialized than in some trilobites, 

 but less so than others. Of the nature of the head-limbs 

 (jaws, etc.) of trilobites we know very little. 



The glabella of Calymene becomes gradually narrower 

 forwards. It ends bluntly, and in front of its end there 

 is a raised border, which margins the whole head-shield, 

 but is slightly arched up in front of the glabella. 



The genae within this raised border are only slightly 

 convex. Each carries a large eye, opposite the first 

 rounded glabella-lobe (third head somite). The eyes are 

 of the compound type, found only in arthropods : instead 

 of one adjustable lens there are many, of fixed curvature, 

 each focusing a small segment of the field of vision on 

 a retinula. The compound character is not well seen in 

 Calymene, as the surface of the eye is smooth. 



On each cheek there is seen a fine dividing line (facial 

 suture] , which starts at the genal angle (outer end of 

 posterior margin of head), runs first obliquely forward, 

 and then transversely, until it reaches the posterior 

 margin of the eye ; here it curves round the inner 

 margin of the eye, and continues straight forward to the 

 front margin of the head-shield, where it continues on 

 the under-side. Just below the margin a transverse 

 suture joins the right and left facial sutures. 



The most probable explanation of the facial suture is 

 that it is a line of easy separation to facilitate the moult- 

 ing (ecdysis) of the head-shield, and especially of the eye, 

 the lenses of which are formed from the cuticle and must 



