The phylogenetic origin of the Tripocalpida may be very divergent, and their morphological affinity to the other #Nassellaria# is a very complicated problem. The Euscenida (genera 504-507) may be derived directly from the Plagonida (_Plagoniscus_) or Plectanida (_Plectaniscus_). The Archibursida (genera 508-510) however, manifest a closer affinity to the Tripospyrida, and may be derived from them by reduction of the sagittal ring and constriction. In these two groups of Archiperida the shell may be regarded as a true cephalis. In the Archipilida, however, where the shell has a wide basal opening and the three radial ribs arise originally from the base of the apical horn, the shell itself may correspond to the thorax of the Dicyrtida, and may be derived from these by loss of the cephalis. This is the more probable, as sometimes a small remnant of the reduced cephalis is preserved (Pl. 98, fig. 8). None of these difficult questions can be answered until a much closer morphological knowledge of the Tripocalpida is acquired. {1135}_Synopsis of the Genera of Tripocalpida._ ------------------------------------------------------------------------- I. Subfamily Archipilida. Basal mouth of the shell a simple wide opening. -------------------- Shell with three lateral ribs or wings, with or without terminal feet. ------------------------------------------------------------------------- { Apex with a horn, 496. _Tripocalpis_. Three terminal feet. { { No apical horn, 497. _Tripodonium_. Six to nine or more } No apical horn, 498. _Tripterocalpis_. terminal feet. } No terminal feet. { Apex with a horn, 499. _Trissopilium_. { { No apical horn, 500. _Archipilium_. ------------------------------------------------------------------------- Shell without lateral ribs or wings but with three terminal feet. ------------------------------------------------------------------------- Feet simple or } Apex with a horn, 501. _Tripilidium_. branched, not latticed. } } No apical horn, 502. _Tripodiscium_. Feet latticed. Apex with a horn, 503. _Tridictyopus_. ------------------------------------------------------------------------- II. Subfamily Archiperida. Basal mouth of the shell closed by a lattice-plate. -------------------- Shell in its axis with a free (simple or branched) internal columella, prolonged into an apical horn. ------------------------------------------------------------------------- Shell with three } Columella simple, 504. _Euscenium_. free feet, without } lateral lattice-wings. } Columella branched, 505. _Cladoscenium_.