CRUSTACEA. TRILOBITA 303 



hypostome, and by the structure of the eyes, as well as by 

 the absence of a genital operculum, and the presence of 

 one pair of eyes only. 



Agnostus. Body small, head-shield and pygidium similar in 

 form and size ; eyes and facial suture absent ; glabella does not 

 reach the anterior border of the head, and has a small lobe at each 

 of the posterior angles. Thorax formed of 2 segments, axis wide, 

 pleurae grooved. Segmentation not shown on the lateral parts of 

 the pygidium. Olenellus Beds to Bala Beds. Ex. A. piriformis, 

 Lingula Flags. 



Microdiscus. Similar to Agnostus but with 3 or 4 segments in 

 the thorax, and axis of pygidium with numerous distinct segments. 

 Olenellus Beds to Lingula Flags. Ex. M. punctatus, Lingula Flags. 



Trinucleus. Head-shield large, with long genal spines, and 

 a broad flat, ornamented border ; glabella inflated, pyriform, furrows 

 sometimes absent. Eyes generally absent. Facial suture absent or 

 indistinct. Thorax formed of 6 segments, pleurae grooved, straight, 

 but slightly curved near their extremities. Pygidium short, tri- 

 angular, margin entire. Arenig to Bala Beds. Ex. T concentricus, 

 Bala Beds. 



Ampyx. Similar to Trinucleus. Head-shield triangular, 

 without a border, and with a long straight spine given off from the 

 front of the glabella ; facial sutures near the external margin, not 

 continuous in front ; free cheeks very narrow. Arenig to Wenlock 

 Beds (chiefly Ordovician, only one species in the Silurian). Ex. A. 

 nudus, Llandeilo Beds. 



Harpes. Resembling Trinucleus, but border of head- shield 

 broader, finely punctate, and extended posteriorly to near the end 

 of the thorax instead of bearing narrow genal spines. Glabella 

 short, convex, not expanded in front. Eyes consist of 2 or 3 lenses, 

 and are usually joined to the front part of glabella by an eye-line. 

 Thorax with 22 to 29 segments ; axis narrow, pleurae long, grooved. 

 Ordovician to Devonian. Ex. H. ungula, Ordovician. 



Paradoxides (fig. 130). Body large, elongated, narrowed 

 posteriorly. Head-shield broad, semicircular, with a border, and 

 long genal spines ; glabella broad in front, with 2 to 4 furrows on 

 each side, some of which are continuous across. Facial sutures 



