224 STUDIES IN EVOLUTION 



side. In this specimen the pyrite which replaced the chiti- 

 nous remains of the animal has decomposed, and the dorsal 

 crust weathered away, exposing below the stems of the exo- 

 podites, with their fringes extending over the entire pleural 

 areas on both sides. A pygidium, with three attached 

 thoracic segments, from another entire specimen (figures 5 

 and 6), preserves the details of the appendages in the most 

 perfect and satisfactory manner. As both halves showed 

 essentially the same extent and disposition of the fringes on 

 the dorsal side, the specimen was cut in two along the centre 

 of the axis, and the left side was then embedded in paraf- 

 fine. By careful preparation the appendages were exposed 

 from the ventral side. 



The cephala of the three specimens described are consider- 

 ably compressed, and from them a very imperfect knowledge 

 of the mouth parts could be obtained, so that this information 

 must be left to future discovery. 



Endopodites. — The three posterior thoracic endopodites 

 are very similar, and in a general way closely resemble those 

 of Triarthrus from the same region of the thorax. They are, 

 however, comparatively shorter and stouter, and could not be 

 extended beyond the ends of the pleura. The two distal 

 joints are cylindrical, with well-marked articular surfaces and 

 ridges. The joints preceding these proximally become much 

 wider, flattened, and produced into transverse extensions 

 which carry large tufts of setae at the end, as also does the 

 end of the last joint of the limb (dactjdopodite). 



The endopodites on the pygidium offer no conspicuous dif- 

 ferences from those just described, except that a gradual 

 change in form is manifest as the terminal limbs are reached. 

 The separate endites become more and more transversely 

 cylindrical, until the whole limb appears to be made up of 

 cylindrical segments transverse to its length. A similar con- 

 dition was observed in the young of Triarthrus* 



Exopodites. — These seem to be composed of slender joints, 

 the distal exites being long and slightly curved outward. 



* Amer. Jour. Sci. (3), XL VII, pi. vii, fig. 3, April, 1894. 



