TEILOBITA. 377 



and the eyes are extremely large and reticulate ; while the 

 segments of the thorax are reduced to five or six in number. 

 15. CHEIRUKID^E. In this large and important family the 

 head-shield is well developed, with discontinuous facial su- 

 tures, which terminate on its outer margin. The glabella 

 is usually highly convex or tumid, with well-marked axal 

 furrows and lateral grooves. There are ten to twelve, gener- 

 ally eleven, body-rings, and the pygidium is small, of from 

 three to five segments, the pleurae terminating in free ends. 

 The family ranges from the Upper Cambrian to the Devonian, 

 but is principally characteristic of the Silurian rocks. In 

 Cheirurus itself (fig. 235, B) the head is semicircular, with 

 rounded or pointed genal angles, and with a strongly-arched 

 glabella, which is deeply grooved by the lateral furrows. 

 There are generally eleven body-rings, with ridged or slightly 

 grooved pleura? ; and the tail has a well-marked axis of 

 four rings, its pleurae being prolonged into points or spines. 

 Amphion (fig. 235, c) is nearly related to Cheirurus, but has 

 from fifteen to eighteen body-rings, and exhibits minor dif- 

 ferences as well. Plaeoparia, again, agrees with Cheirurus 

 in having eleven body-rings, and also in the form and loba- 

 tion of the glabella, but it is destitute of both eyes and facial 

 sutures, as is also the genus Areia. Sphcerexochus, lastly, 

 (fig. 235, D), while agreeing with Cheirurus in its eleven 

 body-rings and in the form of its tail, is distinguished by the 

 extreme inflation of the glabella, and the presence of no more 

 than three segments in the pygidium. We may also place 

 here the very singular and aberrant Silurian genera Stauro- 

 cephalus and Deiphon. In the former of these the general 

 characters correspond with those of Cheirurus, but the 

 anterior or " frontal " portion of the glabella is enormously 

 swollen, and forms a great globular projection in advance of 

 the line of the cheeks. In the still more curious Deiphon 

 the fixed cheeks are rudimentary, and are reduced to two 

 curved spines, which spring from the sides of the swollen 

 glabella, and carry the faceted eyes, while the free cheeks 

 are obsolete. The axis of the tail is formed of four or five 

 rings, and the pleurae are prolonged into two spines on each 

 side, one of these being formed by the first segment only, 



