Sub-Class Trilobita

SUB-CLASS TRILOBITA The tergal plates of the anterior five somites fused to form a dorsal shield or carapace. The preoral appendages long and an tenniform. All the postoral appendages biramous, the external branch branchial, the internal locomotor or also maxillary in func tion. For the other characters and classification see TRILOBITA.

Grade 2. Nomomeristica.

Number of somites never ex ceeding 18 in the adult. Some of the somites of the opisthosoma without appendages. When appendages are present on this area, they are structurally and functionally differentiated from those of the prosoma. The dorsal area, except in some Xiphosura, not trilobite.