There is one pair of appendifers on each of the thoracic segments and four pairs on the pygidium. On the cephalon there is one pair under the neck furrow, and a pair under the posterior glabellar furrows. These are not concealed by the hypostoma. Further forward, and completely covered by the hypostoma, are two much less strongly developed but similar ones, so that there are in all four pairs of appendifers on the cephalon, though it is extremely doubtful if the appendages were articulated directly to all of them. On a specimen of _Ceraurus pleurexanthemus_ 30 mm. long on the median line, the dorsal furrows are 7.5 mm. apart at the anterior end of the thorax, and the tips of the appendifers of this segment are only 4 mm. apart. Each consists of a straight slender rod with a knoblike end projecting directly downward from the dorsal test, and supported by a thin calcareous plate which runs diagonally forward to the anterior edge of the segment directly under the dorsal furrow. On the pygidium three pairs of the appendifers have this form, while the fourth pair consist of low rounded tubercles which are concealed by the doublure. These appendifers are probably cut in many of Walcott's sections of Ceraurus, but owing to the state of preservation it is not always possible to determine what part is appendage, what part is body cavity, and what part is appendifer. Nearly forty years ago Von Koenen (1880, p. 431, pl. 8, figs. 9, 10) described and figured the appendifers of Phacops latifrons. He found them to be calcareous projections on the hinder margin of each segment, converging inward, and about 1.5 mm. long. He correctly considered them as supports (Stützpunkte) for the feet. Appendifers are well developed also in Pliomerops, and in well preserved specimens of _Calymene senaria_ from Trenton Falls they are present, but instead of being rod-like processes, they are rather thick, prominent folds of the shell. They are also well shown in some of the thin sections. A specimen of _Triarthrus_ (No. 229, our pl. 5, fig. 2) has broad processes extending downward from the lower side of the test below the dorsal furrows, much as in _Calymene_, and the individual of _Cryptolithus_ shown in plate 8, figure 1, possesses slender appendifers. Two other specimens (Nos. 237 and 242) show them quite well. They were probably present in all trilobites, but seldom preserved. The appendifers have the same origin as the entopophyses of _Limulus_, and like them, may have relatively little effect on the dorsal surface. _Calymene senaria_ Conrad. (Text figs. 13-16, 23.)