CHAPTER III. 



SYSTEMATIC ARRANGEMENT OP THE SPECIES. 



The arrangement of Trilobites, and their position amongst the Crustacea, now no 

 longer offers any difficulties, and they may be most conveniently described in the following 

 manner. 



If, as the observations already offered would seem to demand, we arrange the genera 

 Cytkerina and Euri/jjfcrus with the Trilobites in a single division, we have a group parallel to 

 the existing PhijUopoda ; to designate which we may employ Dalman's name Palceudce* and 

 which may thus be described. 



The Pal^ad^ are crustaceous Articulata, belonging to the second order of the class 

 Crustacea (divided into Ostracodermata, Aspidoatraca, and Entomostraca), characterized by the 

 possession of two large compound eyes, by the absence of simple secondary eyes, and by 

 having short undeveloped feelers, and soft leaf-formed feet, bearing gills. By these 

 characters they are immediately related to the Phj/IIopoda, and might perhaps even be 

 united with the latter in one tribe. Their principal difference would then consist in the 

 absence of the constant numerical proportion of eleven rings of the thorax, common to ail 

 the FhijUopoda, which must be expressed by the formula of 4 x 3 — 1. Instead of this, the 

 Palceadce exhibit fluctuating numerical proportions in the parts of the thorax, respecting the 

 reduction of which to a constant formula, nothing certain can be said ; because we neither 

 know the number of the accessory parts of the mouth, nor the position of the sexual 

 openings. These animals underwent a progressive metamorphosis, they moved by swim- 

 ming, probably with their backs downwards, and they inhabited the ocean, living chiefly in 

 shallow water. The whole group is divided into three families, which are characterized 

 according to the nature of the shelly covering. 



First Family— EURYPTERID.E. 



In these there is no shell. The head, whose position is very distinct, bears two pair of 

 setaceous feelers, and one pair of accessory parts of the mouth. There are probably nine (?) 

 rings in the thorax, the first of which bears a pair of very large rudder-shaped feet, 



* See the article written by me on the Entomostraca, in Ersch and Gruber's Encyclopiedia, 

 sect, i, vol. xsxv, p. 134. I here first explained the relations of the PalmadtE with the existing 

 Phyllopoda. 



