304 



CRUSTACEA. TRTLOBITA 



extend from the posterior to the anterior border. Eyes large and 

 arched. Thorax long, of 16 to 20 segments ; pleurae grooved and 

 produced into long backwardly-directed spines. Pygidium very 

 small, plate-like, its axis with 2 to 8 segments. Middle Cambrian. 

 Ex. P. davidis, Menevian ; P. bohemicus, Cambrian. 



Oienellus. Similar to Paradoxides 

 same width throughout ; facial 

 sutures not visible ; ' eyes ' large, 

 joined to the glabella ; 13 to 18 

 segments in the thorax. Some 

 forms have spines on the axis 

 of one or more segments of 

 the thorax and pygidium. In 

 some species there are no lateral 

 lobes on the pygidium. Lower 

 Cambrian. 



This genus is divided into 

 four sub-genera : — (1) Oienellus 

 (restricted) ; pygidium spine- 

 like, without lateral lobes. Ex. 

 0. thompsoni. (2) Mesonacis ; 

 pygidium large, with lateral 

 lobes, spines on some segments 

 of the axis of thorax or pygi- 

 dium. Ex. M. vermontana. (3) 

 Holmia ; with a row of spines 

 down the axis of the body — the 

 neck-spine often long ; pygidium 

 small, plate-like. JZx.H.callavei. 

 (4) Ole?ielloides, Ex. 0. armatus. 



Glabella of nearly the 



Fig. 130. Paradoxides davidis, 

 from the Menevian Beds, x £. 



Conocoryphe ( = Conocephalites). Head-shield semicircular, 

 with genal spines (not always preserved) ; axial furrows deep, glabella 

 narrow in front and with 3 or 4 backwardly-directed furrows 

 and well-marked neck-furrow ; free cheeks narrow ; eyes absent. 

 Facial sutures begin just within the genal angles, and cut the front 

 margin. Hypostome convex, formed of a central oval portion sur- 

 rounded by a narrow border. Thorax with 14 or 15 segments ; 

 pleurae grooved. Pygidium small, margin entire, axis with from 



