It is represented either by a spina interna or by a spina externa, or by both, or they join to form a spina communis which is often very large and sometimes ends in a bifurcation.
MESITEs.-Without basipterygoid processes, and with large spina interna.
Without spina interna.
Sub-order Limicolae.-Nidifugous, without spina interna sterni.
Spina externa sterni large, spina interna absent.