306 



CRUSTACEA. TRILOBITA 



the genal angles to the front margin, and often continuous in front. 

 Thorax with 13 segments ; axis wide, not well marked. Pygidium 

 triangular, axis with 10 to 14 segments. Arenig to Devonian. 

 Ex. H. delpkinocepkalus, Wenlock Beds ; H. bisulcatus, Ordovician. 



Ogygla. Body oval, nearly flat. Head-shield large, semi- 

 circular, with a flattened border ; glabella distinct, wider in front, 

 with 4 or 5 lateral furrows. Eyes large. Facial suture passes 

 from the posterior border to the front margin, and is generally con- 

 tinuous at the margin. Free cheeks large. Hypostome not notched. 

 Thorax consisting of 8 segments, axis narrow, distinct ; pleurae 

 grooved, usually with pointed ends. Pygidium large, semicircular, 

 margin entire, axis of numerous segments. Tremadoc to Llandeilo 

 Beds. Ex. 0. buchi, Llandeilo Beds. 



Asaphus (figs. 126, 132). Body oval, surface smooth or with 

 striae. Head-shield large, semicircular with a flattened border, 

 genal angles rounded or spinose ; glabella indistinctly defined, wide 

 in front, with indistinct lateral furrows. Eyes large. Facial 

 sutures pass from the posterior to the anterior margin and are 

 generally continuous at the front margin. Free cheeks large. Hy- 

 postome notched posteriorly. Thorax formed of 8 segments, axis 

 rather broad, pleurae obliquely grooved, with rounded extremities. 

 Pygidium of about the same size as the head, rounded, formed of 

 numerous segments; margin entire. 

 Tremadoc to Bala Beds. Ex. A. ty- 

 rannus, Llandeilo. Sub-genus Asa- 

 phellus : hypostome not notched. 

 Tremadoc Beds. Ex. A . homfrayi. 



Illaenus. Body oval, convex. 

 Head-shield large, semicircular ; 

 glabella indistinctly limited except 

 near the posterior end, without 

 furrows externally. Eyes remote 

 from one another. Facial sutures 

 commence on the posterior border, 

 cut the anterior border in front of 

 eye, and unite on the inferior sur- 

 face. Free cheeks small. Thorax 

 with usually 10 segments, axis 



Fig. 132. Asaphus tyrannus, 



from the Llandeilo Beds. 



x £. 



