Vill CLASSIFICATION 251 
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, 
Deiphon, Placoparia, Sphaerexochus, Amphion, Stawrocephalus. 
Fam. 13. Proétidae (Figs. 150, G; 151, 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 lcbe 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: Proétus, Arethusina, Cyphaspis, 
Phillipsia, 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 
teu 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: Hnerinurus, Cybele, 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. 
1 The British Carboniferous Proétidae are described by H. Woodward, Monogr. 
Brit. Carb. Trilobites, Palaeont. Soc. 1883-84. 
