VIII 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.  Jficrodiscus is apparently 
less modified than Agnostus, on account of the larger 
number of thoracic segments, the more distinct seg- 
mentation of the pygidium, and, in some species, the pyg, 146.—ag- 
larger number of furrows on the glabella. Cambrian = “tus 
ne ; P teger, Beyr., 
and Ordovician. Genera: Agnostus, Microdiscus. x 8 Cam- 
Fam. 2. Shumardiidae.— The body is very small Are 
and oval. The cephalic shield is nearly semicircular 
and very convex, with a broad glabella which expands in front, 
and in which the furrows, except 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 Ampya), 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 Ampyx 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 Zrinuclews no sutures are seen, 
but some examples show indications of what may be a facial 
suture (see p. 226), 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 triangulay. Principally Ordovician. 
Genera: Orometopus (Upper Cambrian), Ampyx, Trinucleus, 
Donde. 
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 
