collar] Fig. 88 A sleeve-like siliceous membrane which is attached to the resting spore mantle in some species, such as C. curvisetus, C. pseudocurvisetus, C. coronatus and some varieties of C. socialis. The sheath can extend in the pervalvar direction further than the spore valve itself does, thus partially obscuring the spore. Either one or both valves may be sheathed. The term collar is not suggested for this structure because it is defined as a projection of the valve face rather than the valve mantle (Crawford 1974), which originates as a vertical extension of the wall of a loculate areola (Crawford 1975).