SUB-CLASS LIMULAVA Resembling the Trilobita in having the prosoma composed of five somites, the preoral appendages antenniform and the append ages of the opisthosoma provided with a locomotor inner and a branchial outer branch.
Order Copura. Nine pairs of appendages on the opisthosoma, none on its last three somites, the terminal somite expanded into a swimming plate but without a postanal caudal spine.
This order containing the family Sidneyidae is represented by genera of mid-Cambrian age. The best known is Sidneyia which in its 12-jointed opisthosoma and absence of "trilobation" of the dorsal surface superficially resembles the Eurypterida. The pos toral appendages of the prosoma are very peculiarly modified.
Resembling apparently the Trilobita and the Limulava in having prosoma five-segmented as the preoral appendages antenniform is a group, the Agalaspina (Agalaspius), of Cambrian age. In cer tain other respects this group shows marked resemblance to the Xiphosura, which follow, and it is considered by Walcott to be the intermediate between that order and the Trilobites. It may for the present be left unclassified.