SUB-CLASS I TRILOBITA 613 
Ellipsocephalus 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 segments 
is large. 
The Pygidium.—The abdomen of Trilobites is commonly known as the 
pygidium (Fig. 1263), 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 limbs, 
marked more or less distinetly by transverse furrows. Sometimes it bears 
considerable resemblance to the cephalic shield (Agnostus, Microdiscus). The 

Fic. 1263. Fic, 1264. 
Pygidium of Ogygia Buchi, Brongt. Ordovician, Pygidium of Bronteus wmbellifer, Beyr. 
Wales. Devonian ; Bohemia. 
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 varies 
considerably in appearance from the thorax.’ 
The axis may extend as far as the posterior end of the pygidium, or to 
any part of the length, but is sometimes reduced to a short rudiment (Lronteus, 
Fig. 1264), or it may be even entirely obscured (Nilews). The number of 
axial segments normally corresponds to the number of pygidial, and varies 
between two and twenty-eight. On the lateral lobes, all or at least a part of 
the pleura may also be seen, being continued from the axis as ribs separated 
by furrows. In these cases, the furrowed and the ribbed pleura can usually be 
distinguished, but not infrequently they have entirely disappeared as surface 
features. Many of the Cambrian Trilobites are conspicuous for their small 
pygidium and elongated thorax. 
The outline of the pygidium is most frequently semicircular, parabolic, or 
elliptical ; more rarely it is triangular or trapezoidal. The margin is entire, 
less commonly dentate or spiny. The border, as in the case of the cephalon 
and the pleura of the segments, has a reflexed margin, or doublure, which in 
some genera attains considerable width. 
The Ventral Side—The ventral side of Trilobites is commonly inaccessible 
for purposes of observation, since, as a rule, it is so firmly attached to the 
