The exopodites in Triarthrus consist of a proximal shaft, succeeded by numerous short segments, and ending distally in a long, grooved, somewhat spatula-shaped segment. Along the anterior margin of the shaft there are many small spines. Along the posterior margin there are numerous flattened setæ, which all lie in one plane and which seem to be more or less united to one another like the barbs of a feather. The setæ are short, not much longer than the width of one of the thoracic segments, and point backward and outward. In _Cryptolithus_ the shaft does not seem to be made up of small segments, and is narrow, with a decided backward curve. The setæ are considerably longer and much more flattened than in Triarthrus. In _Calymene_ the state of preservation does not allow a very full knowledge of the exopodites, but they appear to have a slender, unjointed shaft and short and delicate setæ. The coiled branches of the exopodites as described by Walcott seem to me to be only ordinary Triarthrus-like organs, and this, as I understand from Schuchert, was also the view of Beecher. In _Ceraurus_ the exopodite seems to have been somewhat paddle-shaped, expanded at the distal end, and to have had rather thick, blade-like setæ. The exopodite of _Neolenus_ is decidedly leaf-like, and reminds one somewhat of the exites of some of the phyllopods. The shaft is a broad unsegmented blade. The setæ are slender, delicate, flattened, and a little longer than the width of the shaft. The exopodites of this genus point forward all along the body. In _Kootenia_ the exopodites are like those of _Neolenus_, but with a narrower shaft. The exopodites of _Ptychoparia_ appear to be very much like those of Triarthrus, but the shaft is probably not segmented. The endopodites of the thorax of _Triarthrus_, _Cryptolithus_, and _Acidaspis_ show progressive modification from front to back in the broadening of the individual segments and the assumption by them of a triangular form. Not only do the individual segments become more triangular from front to back, but more of the segments of each endopodite become triangular. This modification has so far been seen in these three genera only. The individual segments, except the distal ones, seem to be flattened in all these genera. The distal end of the terminal segment of each endopodite of _Triarthrus_ bears three small movable spines, and each of the segments usually bears three or more spines, located in sockets along the dorsal surface and at the anterior distal angle of each segment. The endopodite of _Cryptolithus_ is bent backward at the carpopodite and this segment is always thickened. At the distal end of the dactylopodite there is a tuft of spines, the triangular segments have tufts of spines on their posterior corners, and there are groups of spines also in the neighborhood of the articulations.