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 S alter recognised as related to 

 the Olenidae, have been shown by the careful work of Professor 

 Pompeckj l 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' 2 (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 Ncues Jahrb. fur Min. Geol. u. Pal. 1898, i. p. 187. 

 3 Lake, Brit. Cambrian Tril. 1907, p. 45. 



