288 AN INTRODUCTION TO THE STUDY OF FOSSILS 



to the pygidium, and to the ends of the pleurae ; it was further 

 supported by transverse, calcified, chitinous processes to which 

 the legs were attached. 



Triarthrus is well called the daddy longlegs trilobite, for its 

 appendages were unusually long for a trilobite, extending far 

 beyond the margins of the dorsal shield. (In such forms as 

 Trinucleus and Isotelus they did not extend to the margin of the 

 dorsal shield.) These appendages were very uniform in char- 

 acter from the pair directly behind the antennules to the last 

 pair upon the abdomen. 



All except the most anterior pair of appendages, the anten- 

 nules, are biramous, consisting of a basal portion : the protop- 

 odite bearing two branches, — (i) the highly jointed endopo- 

 dite adapted to crawling and (2) the less jointed exopodite, 

 fringed along its posterior border by lamellae-like setae and thus 

 adapted both to swimming and respiration. Anteriorly the 

 endopodites are long, cylindrical, crawling legs; posteriorly 

 they become more and more broadly flattened and leaf-like, 

 bearing tufts of setae ; they are thus well adapted to swimming. 

 Each protopodite has an inward projecting portion, — the jaw- 

 like base or gnathobase. 



There were paired dorsal and ventral muscles beginning in the 

 head and ending in the abdomen ; the dorsal pair extended the 

 animal, the ventral pair flexed it ; but apparently it lacked the 

 power of enrollment (p. 291). The ventral pair of muscles 

 were separated by a chitinous ridge ; from each of these a muscle 

 band extended obliquely outward and backward to the next 

 thoracic segment to be there attached as is indicated by oblique 

 chitinous ridges. The appendages were probably moved simi- 

 larly to those of Apus and Cambarus. 



Judging from the food of the living Crustacea, Triarthrus 

 was carnivorous, eating both decaying and living animal matter. 

 The food brought to the ventral median line of the body by the 

 rhythmical movement of the appendages was seized by the gna- 

 thobases and by them carried forward to the mouth ; the broader 



