148 THE APPENDAGES, ANATOMY, AND RELATIONS OF TRILOBITES. 



need not be included in the classification and for reasons of convenience may be referred 

 to merely as the Protostracean. 



The group of free-swimming trilobites without thoracic segments was probably a large 

 one, and within it there were doubtless considerable variations and numerous adaptations. 

 While the only known animal which could possibly be referred to this group, Naraoia, is 

 blind, it is entirely possible that other species had eyes, and that the cephala and pygidia 

 were variously modified. For this reason and because of the lack of all thoracic segments, 

 it seems better to erect a new order rather than merely a family for the group, and Nek- 

 taspia (swimming shields) may be suggested. The only known family is Naraoidre Wal- 

 cott, which must be redefined. 



Marrella and Habclia are types of Crustacea which can neither be placed in the Trilo- 

 bita nor in any of the established subclasses of the Eucrustacea. They represent a transi- 

 tional 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, Marrellidze Walcott and Aglaspidse Clarke, belong to differ- 

 ent 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, Emeraldclla, and Amiclla 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 antenme 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 antenna?, and possessing facial sutures. 



Order Nektaspia nov. 

 Trilobita without thoracic segments. Cephala and pygidia simple. 



Family Naraoidse 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 antenna;, 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, pygid- 

 ium a small .plate. 



