CLASSIFICATION 245 



the length is greater. Eyes are absent. Facial sutures appear 

 to be absent, but are stated by Beecher to be at the margin of 

 the cephalic shield. From the absence of eyes, 

 the probable absence of facial sutures, the few or 

 indistinct furrows on the glabella, and the smaller 

 number of thoracic segments, the Agnostidae appear 

 to be degenerate forms. Microdiscus is apparently 

 less modified than Agnostiis, on account of the larger 

 number of thoracic segments, the more distinct seg- 

 mentation of the pygidium, and, in some species, the yig. i^Q.—Ag- 

 larger number of furrows on the glabella. Cambrian nostus m- 



° . . teger, Beyr., 



and Ordovician. Genera : Agiiostus, Microdiscus. x 8. Cam- 



Fam. 2. Shumardiidae.— The body is very small B^'I'^^'ji^i^^^®'" 

 and oval. The cephalic shield is nearly semicircular 

 and very convex, with a broad glabella which expands in front, 

 and in which the furrows, excep t the neck-furrow, are indistinct. 

 The facial suture is marginal and eyes are absent. There are six 

 thoracic segments with ridged pleurae ; the axis is broader than 

 the pleurae. The pygidium is large, and is formed of about four 

 segments similar to those of the thorax. Upper Cambrian and 

 Ordovician. Genus : Shumardia. 



Fam. 3. Trinucleidae (Fig. 140). — The head is large and 

 has a flat border (except in Ampyx), and long genal spines. In 

 the earliest genus (Orometopus) the facial sutures start from the 

 posterior margin (near the genal angle) and pass obliquely 

 inwards to the compound eye, from whence they continue 

 forward and unite in front of the glabella. In Amjjyx the 

 suture starts from just within the genal angle and passes to 

 the front border, cutting off a narrow free cheek ; eyes are 

 absent. In most specimens of Trinucleus no sutures are seen, 

 but some examples show indications of what may be a facial 

 suture (see p. 22 6), and a suture is sometimes found at the 

 margin of the cephalic border; eyes may occur (see p. 230). 

 The thorax consists of from five to eight segments, with grooved 

 pleurae. The pygidium is triangular. Principally Ordovician. 

 Genera : Orometopus (Upper Cambrian), Ampi/x, Trinucleus, 

 Dionide. 



Fam. 4. Harpedidae (Figs. 139, G, H; 150, A).— The head 

 is large and has a broad, flat border which is finely punctate, 

 and extends backwards on each side in the form of a horn-like 



