The antennules were obviously tactile organs, probably freely movable in most trilobites, but in the case of Triarthrus perhaps rather rigid, judging from the great numbers of specimens which show the characteristic sigmoid curve made familiar by Professor Beecher's restoration. The proximal end of the shaft of each antennule of Triarthrus is hemispheric and doubtless fitted into a socket, thus suggesting great mobility of the whole organ. In spite of this, I have seen no specimens in which they did not turn in toward each other and cross the anterior margin very near the median line. In front of the margin, various specimens show evidence of flexibility, but from the proximal end to the margin the position is the same in all specimens. In all the few specimens of _Cryptolithus_ retaining the antennules, these organs are turned directly backward, but it is entirely within the range of probabilities that while its burrowing habits made this the more usual position, the animal had the power of turning them around to the front when they could be used to advantage in that direction. _Exopodites._ It has been the opinion of most observers that the exopodites of trilobites were swimming organs, while others have thought that they functioned also in aerating the blood. To the present writer it seems probable that the chief function was that of acting as gills, for which the numerous thin, flattened or blade-like setæ are particularly adapted. That they were also used in swimming is of course possible, but that was not their chief function. It should be remembered that the exopodites are always found dorsal to or above the endopodites, and in a horizontal plane. For use in swimming it would have been necessary to rotate each exopodite into a plane approximately perpendicular to or at least making a considerable angle with the dorsal test. In this position, the exopodites would have been thrust down between the endopodites, and one would expect to find some specimens in which a part at least of the exopodites were ventral to the endopodites. Specimens in this condition have not yet been seen among the fossils. To avoid having the exopodites and endopodites intermingled in this way, the animal would have to bring all the endopodites together along the axial line in a plane approximately perpendicular to the dorsal test, in which case the exopodites would be free to act as swimming organs. The fact that the setæ of an exopodite stay together like the barbs on a feather would of course tend to strengthen the idea that the exopodites could be used in swimming, but that is not the only possible explanation of this condition. The union of the basipodite and exopodite shows that the two branches of the appendage acted together. Every movement of one affected the other, and the motion of the endopodites in either swimming or crawling produced a movement of the exopodites which helped to keep up a circulation of water, thus insuring a constant