vill 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 familes defined by those authors 
have been, in the main, generally adopted. But the phylogenetic 
relationship of the familes 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. Ampycini. Facial sutures obscure, or submarginal, or 
absent. Eyes often absent. Trinucleidae. 
3. Asaphini. Facial sutures ending on the posterior margin. 
Acidaspidae, Lichadidae, Harpedidae, Calymenidae, Paradoxidae, 
Conocephalidae, Olenidae, Asaphidae, Bronteidae, and Proétidae. 
4. Phacopini. Facial sutures ending on the lateral margins. 
Eyes well developed. Phacopidae, Cheiruridae, and Enerinuridae. 
A modification of Salter’s classification has been brought 
forward by Beecher * who divides the Trilobita 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 the Harpedidae. 
! Kingsley does not admit this relationship, 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. 
2 Zittel states that Apus appears first in the Trias. 
3 Monogr. Brit. Trilobites, 1864, p. 2. 
4“ A Natural Classification of Trilobites,” Amer. Jour. Sci. (4), ili., 1897, 
pp. 89-106, 181-207. Reprinted in Beecher’s Studies in Evolution, 1901, p. 109. 
A classification based on the character of the pygidium has been 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 by Lorenz, Zeitschr. d. deutsch. 
geol. Geselisch. \vili., 1906, p. 56. 
