CLASSIFICATION 25 I 



are small, and the eyes usually rather small. There are from 

 nine to eighteen (usually eleven) thoracic segments ; the pleurae 

 have ridges or grooves and free ends. The pygidium is small, 

 consisting of from three to five segments often produced into 

 spines. Upper Cambrian to Devonian. Genera : Cheirurus, 

 Deiplion, Placoijaria, ^^'2)haerexochus, Amphion, Staurocejihahis. 



Fam. 13. Proetidae (Figs. 150, G ; 1 51, A, B). — The body is 

 rather small, and the head forms about a third of its entire length. 

 The glabella is sharply defined, and its furrows are sometimes 

 indistinct ; the posterior furrow curves backward to the neck- 

 furrow, thus limiting a basal lobe on each side of the glabella. 

 The eyes are often large (Fig. 150, G); but in Arethusina (Fig. 

 151, B), in which an eye-line is present, they are small. The 

 facial sutures pass from the posterior to the anterior margin. 

 The free cheeks are large. There are from eight to twenty-two 

 thoracic segments with grooved pleurae. The pygidium is usually 

 formed of numerous segments, and its margin is usually entire. 

 Ordovician to Permian. Genera : Pro'etus, Arethusina, Ct/phasjns, 

 PhillijJsia, Griffithides, Brachymetopus, Dechenella} 



Fam. 14. Encrinuridae. — The cephalic shield is ornamented 

 with tubercles. The free cheeks are narrow, and the eyes very 

 small. The facial suture extends from the lateral margin (or 

 from the genal angle) to the anterior margin. There are from 

 ten to twelve thoracic segments with ridged pleurae. On the axis 

 of the pygidium numerous segments are seen, but usually fewer 

 are indicated on the lateral parts. Ordovician and Silurian. 

 Genera : Encrinurjis, Cyhele, Dindymene. 



Fam. 15. Acidaspidae (Fig. 151, F). — The cephalic shield is 

 broad, with a spinose margin, genal spines, and sometimes spines 

 on the neck-ring. The glabella has a longitudinal furrow on 

 each side, due to the backward bending of the lateral furrows. 

 The facial suture passes from the posterior border (near the genal 

 angle) to the anterior border. The free cheeks are large ; the 

 eyes small. There are from eight to ten thoracic segments with 

 ridged pleurae, which are produced into long backwardly directed 

 spines. The pygidium is short, and is formed of two or three seg- 

 ments with long spines at the margin. Ordovician to Devonian. 

 Genus : Acidaspis. 



^ The British Carboniferous Proetidae are described by H. Woodward, Mono(jr. 

 Brit. Curb. Trilobites, PaUieont. Soc. 1883-84. 



