CLASSIFICATION 243 



it probable that they had some ancestral connexion ; ^ the 

 possibility of such a relationship receives some support from 

 the presence in the Lower Cambrian rocks of Protocaris, a 

 genus of the Phyllopoda which resembles Apus.^ The primitive 

 characters of Trilobites are the variable and often large number 

 of segments in the thorax and pygidium ; the presence of a pair 

 of appendages on every segment except the anal ; the biramous 

 form of all except the first pair of appendages ; and the lack 

 of specialisation shown by the appendages, especially those of 

 the head. 



The classification of Trilobites is due largely to the work of 

 Barrande and Salter, and the families defined by those authors 

 have been, in the main, generally adopted. But the phylogenetic 

 relationship of the families has still, to a large extent, to be 

 established. Salter^ arranged the families in four groups, but 

 did not claim that that classification was entirely natural. His 

 groups with the families included in each are : — 



1. Agnostini. Without eyes or facial suture. Agnostidae. 



2. Ainjjycini. Facial sutures obscm^e, or submarginal, or 

 absent. Eyes often absent. Trinucleidae. 



3. Ascvphini. Facial sutures ending on the posterior margin. 

 Acidaspidae, Lichadidae, Harpedidae, Calymenidae, Paradoxidae, 

 Conocephalidae, Oleuidae, Asaphidae, Bronteidae, and Proetidae. 



4. Phacojnni. Facial sutures ending on the lateral margins. 

 Eyes well developed. Phacopidae, Cheiruridae, and Encrinuridae. 



A modification of Salter's classification has been brought 

 forward by Beecher ■* who divides the Trilol)ita into three main 

 groups : — 



1. Hypoparia. Facial sutures at or near the margin, or 

 ventral. Compound eyes absent. This is equivalent to Salter's 

 Agnostini and Ampycini with the addition of tlie Harpedidae. 



^ Kingsley does not admit tliis relationsliip, and regards the Trilobita as a group 

 quite distinct from all other Crustacea. See American Naturalist, xxviii., 1894, 

 p. 118, and American Geologist, xx., 1897, p. 33. 



" Zittel states that Ai^us ap])ears first in the Trias. 



3 Monogr. Brit. Trilobites, 1864, p. 2. 



* " A Xatural Classification of Trilobites," Amer. Jour. Set. (4), iii., 1897, 

 pp. 89-106, 181-207. Reprinted in Beecher's Stiidies in Evolution, 1901, p. 109. 

 A classification based on the chai-acter of the pygidium has beon proposed by 

 Giirich, Centralbl. fiir Min. Geol. u. Pal. 1907. p. 129. A classification based on 

 the minute structure of the test has been given liy Lorenz, Zeitschr. d. deutscli. 

 geol. Gesellsch. Iviii., 1906, p. 56. 



