TRILOBITA. 



537 



Family 9. JEglwidje. — This family contains only the single 

 genus stEglina (fig. 395, a), of the Ordovician rocks, and is chiefly 

 distinguished from the preceding by the much larger size of the 

 eyes, and the smaller number of body-rings. The head and tail are 

 both of large size, the latter with a truncated axis ; the glabella is 

 not conspicuously marked out ; the facial sutures are discontinu- 

 ous ; and the eyes are extremely large and reticulate ; while the 

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



Family 10. Cheirurid^. — In this large and important family 

 the head-shield is well developed, with discontinuous facial sutures, 

 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, rarely fifteen or eighteen, generally 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 Ordovician and Silurian rocks. In Cheirurus itself (figs. 395, b, 

 and 396) 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 pleurae ; and the tail has 

 a well-marked axis of four rings, its pleurae 

 being prolonged into points or spines. 

 Amphion (fig. 395, c) is nearly related to 

 Cheirurus, but has from fifteen to eighteen 

 body-rings, and exhibits minor differences 

 as well. Placoparia, again, agrees with 

 Cheirurus in having eleven body-rings, and 

 also in the form and lobation of the gla- 

 bella, but it is destitute of both eyes and 

 facial sutures, as is also the genus Areia. 

 Sphcerexochus, lastly (fig. 395, d), while 

 agreeing with Cheirurus generally, is dis- 

 tinguished by the extreme inflation of the 

 glabella, and the presence of no more than 



three segments in the pygidium ; while the basal lobes of the gla- 

 bella are completely isolated, and there are only ten body-rings. 

 We may also place here the very singular and aberrant genera 

 Staurocephalus 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. 

 The species of Staurocephalus are Ordovician and Silurian. In the 



Fig. 396. — Cheirurus insignis, 

 from the Silurian rocks of Bo- 

 hemia. (After Barrande.) 



