Corda (1847) made a number of claims for appendages, but all were shown by Barrande (1852) to be erroneous. Barrande (1852, 1872) gave a somewhat incomplete summary of the various attempts to describe the appendages of trilobites, concluding that none showed any evidence of other than soft appendages, until Billings' discovery of 1870. Volborth (1863) described a long chambered tubular organ in _Illænus_ which he believed to represent a cast of the heart of a trilobite, but which has since been likened by writers to the intestinal tract in "_Trinucleus_." PART I. THE APPENDAGES OF TRILOBITES. Terminology. The terminology employed in the succeeding pages is essentially the same as that used by Beecher, with two new terms added. Beecher assigned to the various segments of the limbs the names suggested by Huxley, but sometimes used the name protopodite instead of coxopodite for the proximal one. It is obvious that he did not use protopodite in the correct sense, as indicating a segment formed by the fusion of the coxopodite and basipodite. The usage employed here is shown in figure 1. [Illustration: Fig. 1.--_Triarthrus becki_ Green. Diagram of one of the limbs of the thorax, viewed from above, with the endopodite in advance of the exopodite. 1, coxopodite, the inner extension being the endobase (gnathobase on cephalon); 2, basipodite, springing from the coxopodite, and supporting the exopodite, which also rests upon the coxopodite; 3, ischiopodite; 4, meropodite; 5, carpopodite; 6, propodite; 7, dactylopodite, with terminal spines.] The investigation of _Ceraurus_ showed that the appendages were supported by processes extending downward from the dorsal test, and on comparison with other trilobites it appeared that the same was true in _Calymene_, _Cryptolithus_, _Neolenus_, and other genera. Thin sections showed that these processes were formed by invagination of the test beneath the dorsal and glabellar furrows. While these processes are entirely homologous with the entopophyses of _Limulus_, I have chosen to apply the name _appendifer_ to them in the trilobites. The only other new term employed is the substitution of _endobase_ for gnathobase in speaking of the inner prolongation of a coxopodite of the trunk region. The term gnathobase implies a function which can not in all cases be proved. The individual portions of which the limbs are made up are called _segments_, and the articulations between them, _joints_. Such a procedure is unusual, but promotes clearness. The Appendages of Neolenus. HISTORICAL.