THE TRILOBITA AND OTHER ARTHROPODA 193 



The limbs or appendages of Arthropods are usually of 

 the greatest importance for classification. Unfortu- 

 nately the limbs of trilobites are very rarely found. 

 The ventral exo-skeleton of aquatic Arthropods has 

 always much more thin cuticle and smaller sclerites 

 than the dorsal, but in trilobites it seems to have been 

 nearly all thin cuticle, which easily decayed and allowed 

 the limbs, themselves soft, to be lost. 



Calymene Uumenbachi is sometimes found lying flat, 

 sometimes rolled-up exactly in the way in which the 

 modern wood-louse rolls itself up when alarmed that is 

 to say, with the ventral sides of head-piece and tail-piece 

 in close contact (compare Fig. 59, e). Comparison of 

 specimens in the two conditions shows that neither 

 head nor pygidium is flexible, but that the somites of 

 the intermediate region (commonly called the thorax) are 

 movable on one another by pivots at the sides, their 

 dorsal portion sliding partly over one another when the 

 animal stretched itself out, and becoming fully exposed 

 when it rolled up. 



In an ordinary-sized specimen 50 mm. long, the head 

 measures about 16 mm. in length, the thorax 27 mm., 

 and the pygidium 7 mm. Subsequent measurements 

 refer to such an one, but the species sometimes attained 

 nearly double these dimensions. 



Each thoracic or free somite consists of a central arched 

 portion (axis) and a pair of lateral pleura. The axis of 

 the first somite is about 1 1 mm. wide, that of the last 

 about 8 mm. : each consists of a prominent arch, measur- 

 ing 2 mm. from front to rear, and an anterior sunken 



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