Vil THORAX AND ABDOMEN 235 
succeeding pleurae; such occurs principally in pleurae with 
ridges, as in Cheirurus and Lronteus; or (ii1.) 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 Phacops, Calymene, Sao, Asaphus, 
Llipsocephalus. 
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. 
Tn 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. 157, A, 3) 
are similar to those of the thorax, except that they are fused 
together. In a few forms, such as J//aenus (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 Enerinurus. 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 
1 For an example of this see Salter, Mon. Brit. Trilobites, 1864-83, pls. 
15, 16. 
