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.[190] 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 _Illaenus_ (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 segments 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 pygidium radiate from it in a fan-like manner. Occasionally, as in _Bumastus_, the axis cannot be distinguished from the lateral parts. In a few early Trilobites (_Olenellus_, _Holmia_, Fig. 148, _Paradoxides_, Fig. 147) the lateral parts of the pygidium are very small. In some genera, such as _Asaphus_, the marginal part of the pygidium forms a flattened or concave border. The margin may be entire or produced into spines, and sometimes (Fig. 151, C) a caudal spine comes off from the end of the axis. On the ventral surface of the pygidium there is a marginal rim similar to the doublure of the cephalic shield. The anus is on the ventral surface of the last segment of the pygidium.