=Fam. 1. Agnostidae= (Fig. 146).—Small Trilobites, in which the head and pygidium are of nearly the same size and shape. The thorax is shorter than the head or pygidium, and consists of from two to four segments with grooved pleurae. The length and width of the head are commonly nearly equal, but sometimes 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 _Agnostus_, on account of the larger number of thoracic segments, the more distinct segmentation of the pygidium, and, in some species, the larger number of furrows on the glabella. Cambrian and Ordovician. Genera: _Agnostus_, _Microdiscus_. =Fam. 2. Shumardiidae.=—The body is very small 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 _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 _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 _Trinucleus_ 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 triangular. Principally Ordovician. Genera: _Orometopus_ (Upper Cambrian), _Ampyx_, _Trinucleus_, _Dionide_.