ARTHROPODA phylum vii 



698 



theaxis. and towards the free ^^^^^^ ^SetZcÄ^; || 



^^V;.r It^tlVoTÄ^^^^^^^^^^^^^ 0. inner poHion The 



Ivtte^ exende rom the axis to the fulcrum or bend i... to a p ace where the 

 nleu a bend more or less abruptly downward, and aUo genera y toward the 

 ^ear The dktal portion, beginning at the Merum, may continue o£ equal 

 Ihkknel and be rounded or obtuse at the extremity, or it may decrease :n 



^'"xt nrl:ro;"thri: seg^ents dürers exceedingly among different 

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

 fo £ar observed, twenty-nine, is found in some spemes of Harpes. A Variation 

 L ^ bfnoted even anfong the .pecies of a single genu. hence this ehar^cter 

 is not of general application for purposes of Classification. For example, 



Fig. 1339. Fi«- l^*^- 



PvL'idium of Onygiocaris huchi (Brongt.). Pygidium of Goldüis umhellißr (Beyr.). 



Ordovician, Wales. Devonian ; Boliemia. 



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

 Fhillipsia with nine to fifteen, Cheimrus with ten to twelve, Cyphaspis with ten 

 to seventeen, Ellipsoceplialus with ten to f ourteen, 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, Eodiscus). 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 differs 

 considerably in appearance from the thorax. 



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



