_Marrella_ and _Habelia_ are types of Crustacea which can neither be placed in the Trilobita nor in any of the established subclasses of the Eucrustacea. They represent a transitional group, the members of which are, so far as known, adapted to the crawling mode of life, though it may prove that there are also swimmers which can be classified with them. To this subclass the name _Haplopoda_ may be applied, the feet being simple. The two known families, Marrellidæ Walcott and Aglaspidæ Clarke, belong to different orders, the second having already the name Aglaspina Walcott. The name _Marrellina_ may therefore be used for the other. For _Sidneyia_, Walcott proposed the new subordinal name Limulava, placing it under the Eurypterida. While _Sidneyia_, _Emeraldella_, and _Amiella_ may belong to the group that gave rise to the Eurypterida, they are themselves Crustacea, and a place must be found for them in that group. The possession of only one pair of antennæ prevents their reception by the Haplopoda, and allies them to the Trilobita, but the modifications of the trunk and its appendages keep them out of that subclass, and a new one has to be erected for them. This may be known as the _Xenopoda_, in allusion to the strange appendages of _Sidneyia_. _Synopsis._ Class Crustacea. Subclass Trilobita Walch. Crustacea with one pair of uniramous antennæ, and possessing facial sutures. Order Nektaspia nov. Trilobita without thoracic segments. Cephala and pygidia simple. Family Naraoidæ Walcott. Cephalon and pygidium large, both shields nearly smooth. Eyes absent. A single species: _Naraoia compacta_ Walcott, Middle Cambrian, British Columbia. Subclass Haplopoda nov. Crustacea with trilobate form, two pairs of uniramous antennæ, no facial sutures, sessile compound eyes present or absent, pygidium and pleural lobes generally reduced, large labrum present, appendages of the trunk biramous. Order Marrellina nov. Form trilobite-like, pleural lobes reduced, endobases absent from coxopodites of body, pygidium a small plate. Family Marrellidæ Walcott. Cephalon with long genal and nuchal spines. Eyes marginal. A single species: _Marrella splendens_ Walcott, Middle Cambrian, British Columbia. Order Aglaspina Walcott. Body trilobite-like, with few thoracic segments, and a spine-like telson. Appendages biramous. Family Aglaspidæ Clarke. Cephalon trilobate, with or without compound eyes, seven or eight segments in the thorax. Genus _Aglaspis_ Hall. Compound eyes present, seven segments in thorax. Upper Cambrian, Wisconsin. Genus _Molaria_ Walcott. Compound eyes absent, eight segments in thorax. Middle Cambrian, British Columbia. Genus _Habelia_ Walcott. Compound eyes absent. Not yet fully described. Middle Cambrian, British Columbia. Subclass Xenopoda nov. Crustacea with more or less eurypterid-like form, one pair of uniramous antennæ, biramous appendages on anterior part of trunk, modified endopodites on cephalon.