TRILOBITA. 



541 



facial sutures being separate, and the eyes sickle-shaped. There 

 are ten thoracic segments, and the pleurae are ridged. 



Family 16. Phacopid^e. — In this, one of the best-marked and 

 most typical of the families of the Trilobites, the head is well de- 

 veloped, the glabella conspicuously broadest in front, with three or 

 four lateral grooves, and the facial sutures united in front of the gla- 

 bella, and cutting the outer margins of the cephalic buckler behind. 

 The eyes are usually large, and are faceted (fig. 403) ; there are 

 eleven body-rings, with grooved pleurae ; and the condition of the 

 pygidium is variable. The hypostome is convex, and more or less 



Fig. 402. — a, PhacnpsStern- 

 bcrgi, from the Silurian rocks 

 of Bohemia ; B, Head-shield 

 of Phacops ( Trimerocephalus) 

 Volborthi, Silurian, Bohemia. 

 (After Barrande. Copied from 

 Zittel.) 



Fig. 403. — Phacops (Dal- 

 manites) longicaudata. Silu- 

 rian (Wenlock) of Britain and 

 North America. 



triangular in shape. The genus Phacops itself (with the sub-genera 

 Trimerocephalus, Phacops proper, Acaste, Pterygometopus, Chasmops, 

 Dalmanites, and Cryphceus) constitutes the entire family, and ranges 

 from low down in the Ordovician series to the Upper Devonian 

 rocks. 



As regards the sub - genera of Phacops, Trimerocephalus includes 

 Silurian and Devonian forms, in which the glabella is tumid in front 

 (fig. 402, b) ; the glabella-furrows are faint or wanting ; and the pygidium 

 is of small size. In Phacops proper (fig. 402, a) the genal angles of the 

 head-shield are rounded ; the two anterior pairs of glabella-furrows are 

 inconspicuous, and the glabella is very wide in front ; the pleurae are 

 rounded ; and the pygidium is of few segments, with an entire margin. 



