vni THORAX AND ABDOMEN 235 



succeeding pleurae ; such occurs principally in pleurae with 

 ridges, as in Cheirurus and Bronteus ; or (ii.) it may be in the 

 form of a long cylindrical process, as in many species of Acidaspis ; 

 or (iii.) the external part may be of the same width, either 

 throughout or in part, as the internal part, and may overlap the 

 next pleura behind ; this type is found principally in pleurae 

 with a groove such as in Fhacops, Calymene, Sao, Asaphus, 

 Ullipsocephahis. 



In some Trilobites there is beyond the fulcrum a smooth, 

 flat, triangular part at the front margin of the pleura ; this part 

 is known as the " facet," and forms a surface articulating with 

 the preceding segment which overlaps it. 



In the remarkable form Deiphon (Fig. 151, E) the pleurae are 

 separate throughout their entire length. 



In some Trilobites broad and narrow forms of the same species 

 occur — the difference being seen especially in the axis. The 

 former are regarded as females, the latter as males.^ 



The segments of the abdomen or pygidium (Fig. 137, A, 3) 

 are similar to those of the thorax, except that they are fused 

 together. In a few forms, such as Ulaenus (Fig. 150, F) 

 and Bumastus, the fusion is so complete that no trace of 

 segmentation can be seen on the dorsal surface. Usually, 

 however, the segments are easily distinguishable ; the number 

 seen on the axis is commonly greater than on the lateral 

 parts of the pygidium ; this difference is particularly well 

 shown in Encrinurus. In Trilobites which have grooved 

 pleurae the conspicuous grooves seen on the lateral parts 

 of the pygidium are the grooves of the pleurae, the sutures 

 between the pleurae being less distinct. The shape of the 

 pygidium may be semicircular, a segment of a circle, trapezoidal, 

 triangular, semi-parabolic, etc. ; its size varies considerably ; in 

 the Cambrian forms it is usually small, but in the Trilobites of 

 later periods it becomes relatively larger. The number of seg- 

 ments in the pygidium varies from two to twenty-eight. The 

 axis of the pygidium tapers more rapidly than that of the thorax; 

 sometimes it reaches quite to the posterior end of the body, but 

 is commonly shorter than the pygidium ; in Bronteus it is 

 extremely short, and the grooves on the lateral parts of the 



I For an example of this see Salter, 3fon. Brit. TriloUtes, 1864-83, pis. 

 15, 16. 



