698 



ARTHROPODA 



PHYLUM VII 



the axis, and towards the free extremity; and (2) ridged pleura {jplhvre a 

 hourrelet), having a longitudinal ridge or narrow fold. These characters vary- 

 considerably, and are sometimes obscure. In a small number of genera 

 (Illaemis, Nileus) the pleura are perfectly smooth. 



All pleura show a distal or lateral, and a proximal or inner portion. The 

 latter extends from the axis to the fulcrum or bend, i.e. to a place where the 

 pleura bend more or less abruptly downward, and also generally toward the 

 rear. The distal portion, beginning at the fulcrum, may continue of equal 

 thickness and be rounded or obtuse at the extremity, or it may decrease in 

 size and terminate in a spine. 



The number of thoracic segments differs exceedingly among different 

 genera. The smallest number, two, occurs in Agnostus. The largest number 

 so far observed, twenty-nine, is found in some species of Harpes. A variation 

 is to be noted even among the species of a single genus, hence this character 

 is not of general application for purposes of classification. For example. 



Fig. 1339. 



Pygidiuni of Oiiygiocaris huchi (Brongt.). 

 Ordovician, Wales. 



Fig. 1340. 



Pygidium of Goldius umbellifer (Beyr.). 

 Devonian ; Bohemia. 



there are species of Ampyx and Aeglina with five to six thoracic segments, 

 Phillipsia with nine to fifteen, Cheirurus with ten to twelve, Cyphaspis with ten 

 to seventeen, ElUpsocephahis with ten to fourteen, and Paradoxides with sixteen 

 to twenty. In general, there seems to be a sort of mutual relationship 

 between the number of thoracic segments and the size of the pygidium. 

 When the latter is large, the thoracic segments are usually few ; but if small, 

 the number of thoracic segments is large. 



The Pygidium. — The abdomen of Trilobites is commonly known as the 

 pygidium (Fig. 1339), though sometimes styled the caudal shield or plate. It 

 consists of a single piece, with an arched upper surface, upon which may be 

 distinguished regularly a median axis and two lateral parts, or pleural lobes, 

 marked more or less distinctly by transverse furrows. Sometimes it bears 

 considerable resemblance to the cephalic shield {Agnostus, Eodiscm). The 

 pygidium evidently originated from the anchylosis of a number of similar 

 segments. The potential segmentation is often so strongly marked that it is 

 very difficult to recognise the dividing line between the thorax and pygidium, 

 except in disarticulated specimens. Sometimes the evidences of segmentation 

 disappear entirely or are but faintly indicated on the lower side. When 

 segmentation along the axial and lateral lobes is weak, the pygidium difl'ers 

 considerably in appearance from the thorax. 



The axis may extend as far as the posterior end of the pygidium, or to 



