202 ARTHROPODA. TRILOBITA. 



Calymene. Head-shield semicircular, genal angles 

 rounded, occasionally pointed ; glabella greatly inflated, 

 broadest behind, with three pairs of lateral furrows, form- 

 ing three globular lobes on each side. Eyes prominent. 

 Facial suture extending from the genal angle to the 

 anterior border. Thorax formed of thirteen segments, 

 axis prominent, pleurae grooved and facetted. Pygidium 

 with six to eleven segments, margin entire. Arenig to 

 Upper Ludlow. 



Homalonotus. Body large, elongated, trilobation 

 indistinct. Head-shield broad, genal angles rounded, 

 furrows on the glabella indistinct or absent. Eyes small. 

 Facial suture passing from the genal angles to the front 

 margin, but often continuous in front. Thorax with 

 thirteen segments ; axis wide, not well-marked. Pygidium 

 rather small, triangular, axis with ten to fourteen seg- 

 ments. Arenig to Devonian. 



Ogygia. Body large, nearly flat. Head-shield large, 

 semicircular; glabella distinct, wider in front, with four 

 lateral furrows. Eyes large. Hypostorne not notched. 

 Facial suture starting from the posterior border, sometimes 

 cutting the front margin, but generally continuous. Tho- 

 rax consisting of eight segments, axis narrow, distinct; 

 pleurae grooved. Pygidium large, semicircular, margin 

 entire, axis of numerous segments. Tremadoc to Llan- 

 deilo Beds. 



Asaphus. Body oval, surface smooth or with striae. 

 Head-shield semicircular, with a flattened border, genal 

 angles rounded or spinose ; glabella wide in front and 

 usually without lateral furrows. Eyes large. Hypostome 

 notched posteriorly. Thorax formed of eight segments, 

 pleurae obliquely grooved, with rounded extremities. 



