BUB-OLAM I TIMI.oniTA 



with ten to fourteen, and /'//-i///...r/W.> with -ixteen to t wentv. 

 In general, there seems to be a sort of mutual relationship between tin- numl er 

 of thoracic segments and the si/e of the pygidium. When tin- lattci i> lar^e. 

 the thoracic segments are usually few : hut if small, the number of segments 

 is large. 



The Pygidiwn. The abdomen of Trilobites is common Iv known as the 

 /tftffiilium (Fig. 1263), though sometimes styled the caudal shield or plate. It 

 consists of a single piece, with an arched upper surface, upon which may 1 e 

 distinguished regularly a median axis and two lateral parts, or pleural limbs, 

 marked more or less distinctly by transverse furrows. Sometimes it 

 considerable resemblance to the cephalic shield (./'///>///>, .]//>////>/.<). The 



FIG. 1263. Fi;. 



Pyjiiclium of Ogygia Buchi, Brongt. Ordovician. Pygidium of /,';<;//(."> miileU'ifer, 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. AVhen 

 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 ( /;/'<>/if> n.--. 

 Fig. 1264), or it may be even entirely obscured (Nileus). 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 



