The existence of some relationship between the Trilobita and the Leptostraca (Phyllocarida) has been maintained by Professor G. H. Carpenter.[195] He points out that some of the earliest Trilobites, such as _Holmia kjerulfi_ (Fig. 148), possess nearly the same number of segments as _Nebalia_ (Fig. 76, p. 111), and that in the latter genus the cephalic appendages, especially the mandibles and maxillae, are less specialised than in _Apus_, and consequently differ less from those of Trilobites than do the appendages of the Apodidae. Further, in another genus of the Leptostraca, _Paranebalia_, the biramous thoracic legs, in which both endopodite and exopodite are elongate, approach those of Trilobites more nearly than do the thoracic legs of _Apus_. The view[196] that some connexion may exist between the Isopoda and the Trilobita seems to have been based on the similar dorso-ventral flattening of the body, its division into three regions—head, thorax, and abdomen—and the presence of sessile eyes. Beyond this it is difficult to find any resemblance; whilst the differences, such as the variable number of thoracic segments and their biramous appendages in Trilobites, are important. At present, then, we can only conclude that the Trilobita are more primitive than any other Crustacea, and that their resemblance to some of the Phyllopoda is sufficient to make it probable that they had some ancestral connexion;[197] 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_.[198] 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[199] 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 Proëtidae. 4. _Phacopini._ Facial sutures ending on the lateral margins. Eyes well developed. Phacopidae, Cheiruridae, and Encrinuridae.