244 TRILOBITA CHAP. 
2. Opisthoparia. Facial sutures extending from the posterior 
margin to the front margin, but occasionally uniting in front 
of the glabella. Eyes holochroal or prismatic, but sometimes 
absent. This comprises the same families as Salter’s Asaphini 
with the exclusion of the Harpedidae and Calymenidae. 
3. Proparia. Facial sutures extending from the lateral 
margins, and either cutting the anterior margin or uniting in front 
of the glabella. Eyes holochroal or schizochroal; occasionally 
absent. This is equivalent to Salter’s Phacopini with the 
addition of the Calymenidae. 
In each of the groups proposed Beecher regards as the more 
primitive forms those which possess characters similar to those 
of the early larval stages, such as narrow free cheeks, the absence 
of compound eyes, and a glabella which is broad in front and 
reaches the anterior margin of the head. 
The modifications introduced by Beecher can scarcely be re- 
garded as making Salter’s classification more natural. For instance, 
the Agnostidae differ so much from all other families that, at 
present, there is no evidence to show that they have any close 
phylogenetic relationship with the Trinucleidae and Harpedidae. 
Further, the Calymenidae, which Salter recognised as related to 
the Olenidae, have been shown by the careful work of Professor 
Pompeckj ' to have descended from the latter family, and to have 
no genetic connexion with the Phacopidae with which they are 
grouped by Beecher. Then in the Trinucleidae the earliest genus, 
Orometopus” (Fig. 140, A), possesses compound eyes and facial 
sutures which begin at the posterior margin and unite in front 
of the glabella ; so that, according to Beecher’s classification, that 
genus would belong to the Opisthoparia, whereas the later genera 
(Trinucleus, etc.) of the same family would be placed in the 
Hypoparia. At present, therefore, the only classification of 
Trilobites which can be adopted is a division into families, of 
which a short account is given below. 
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 
1 Neues Jahrb. fiir Min. Geol. u. Pal. 1898, i. p. 187. 
2 Lake, Brit. Cambrian Tril. 1907, p. 45. 
