No. 634] PHYLOGENY OF THE ARTHR OPODA 399 



(coxopodite) supports the proximal ends of two branches, 

 an ambulatory endopodite and a setiferous respiratory 

 exopodite. The endopodite consists in all cases of six 

 segments, the terminal one with movable spines on the 

 distal end, usually three in number, but occasionally sev- 

 eral. The proximal segment of the endopodite is a basi- 

 podite and gives rise to the exopodite, although both 

 branches articulate with the coxopodite. The method of 

 articulation of coxopodite, exopodite, and basipodite is 

 similar to that of the second thoracic limb of the recent 

 Anaspides, as figured by Caiman. The exopodite is in all 

 cases composed of a flattened shaft, along the posterior 

 margin of which are delicate flattened seta?. The form 

 of articulation of the basipodite, exopodite, and coxo- 

 podite indicates that when one of the outer branches 

 moved the other accompanied it, but as the exopodites 

 were always above the endopodites, they appear to have 

 been of comparatively little use in swimming, and were 

 probably chiefly respiratory organs. 



All of the trilobites mentioned, so far as their state of 

 preservation will allow determination, have four pairs of 

 limbs of this sort on the cephalon, a pair on each segment 

 of the thorax, and as many pairs on the pygidial shield 

 as there are annulations on its axial lobe. In front of 

 the biramous limbs there is one pair of uniramous, richly 

 segmented, tactile antennules. The ventral membrane of 

 the trilobite was very thin and feebly supported, so that 

 the articulation of the limbs was not with it, but with 

 infoldings of the dorsal shell which extended downward 

 beneath the glabeller and dorsal furrows. The distal end 

 of each of these appendifers fitted into a notch in the 

 upper side of the corresponding coxopodite. A projec- 

 tion of this latter segment extended mesally nearly to the 

 median line, forming endobases which on the cephalon, 

 and usually along the whole of the body, functioned as 

 food-getting organs. 



No other parts of the limbs have yet been found, al- 

 though I have searched diligently through all of the 



