200 ARTHROPODA. TRILOBITA. 



the resemblance is more marked, since some of these 

 possess facial sutures and have the segments of the thorax 

 and abdomen free. The larval form of Limulus presents 

 a great resemblance to some trilobites, in this stage the 

 caudal spine is absent and the segmentation is better 

 marked. The trilobites differ from the Xiphosurans in 

 the absence of ocelli, in the possession of a hypostome, and 

 in the form and arrangement of the appendages. 



Agnostus. Body small, head-shield and pygidium 

 similar in form and size ; eyes and facial suture absent. 

 Thorax formed of two segments, pleurae grooved. Harlech 

 to Bala Beds. 



Microdiscus. Similar to Agnostus but with four 

 segments in the thorax. Harlech to Lingula Flags. 



Trinucleus. Head -shield large, with long genal 

 spines, and a broad punctate border; glabella inflated, 

 pyriform, furrows absent, or indistinct. Eyes generally 

 absent. No facial suture. Thorax formed of six seg- 

 ments, pleurae grooved, straight, but slightly curved near 

 their extremities. Pygidium short, triangular, margin 

 entire. Arenig to Bala Beds. 



Ampyx. Similar to Trinucleus, but without a punc- 

 tate border to the head-shield, and with a long straight 

 spine given off from the front of the glabella, and also 

 with discontinuous facial sutures. Tremadoc to Bala 

 Beds. 



Olenus. Body oval ; head-shield larger than the 

 pygidium, with a narrow border, and genal spines; glabella 

 usually a little contracted in front, separated from the 

 anterior border by a flat space ; facial sutures discontinu- 

 ous in front; eyes well-developed, placed forward and 



