The aboral surface of Spiraclavus nacoensis. PR = peripheral rim, RZ = recumbent zone, PZ = pedunculate zone.

Plates forming the recumbent zone are positioned between the thecal edge and pedunculate zone on the aboral surface. Typically the recumbent zone is composed of several circlets of plates that increase in size distally. In Clavidiscus this zone is absent (Sumrall, 1996) and in Giganticlavus this zone is 12 mm wide (Sumrall and Bowsher, 1996). In some discocystinids the zone is narrow and the distal plate circlets are aligned with the peduncular columns whereas in others the zone is much wider and the distal circlets of plates are irregular with respect to the peduncular columns. The recumbent zone plates are typically the same thickness as the interambulacral plates and usually have semimbricate sutures, but in Giganticlavus the sutures are fully adjacent.

Left, exterior view of a recumbent zone plate from Spiraclavus nacoensis. Right, interior view of same plate. P = suture where pedumculate zone attaches, I = imbricate sutures where other recumbent zone plates attach.