In Megacystis the periproct—which S. A. Miller always called the moutn—lies between adoral circlets II and III, being bounded by the two posterior facet-bearing plates and the posterior interradial, i.e. 3 of Adorals II, also by 2 or 3 of Adorals III, making 5 of 6 plates in all. The number 5 is the more usual; it occurs in M. aspera. faberi, gorbyi, indianensis, ornatissima, ornata, parcula, parca, perlonga, plena, scitulus, and spangleri, also in British Museum specimens E 7631, —33, —35, —37, —38, ?—39, —40, —42, —44, —45, —74, —76, —77, E16167, and E 16168 (see our figs. 24, 25, antea). The number 6, due to an additional Adoral III, is found in M. baculus, commoda, splendens, and perhaps gyrinus, also in E7630, —34, —36, ?—75, and E 16171 (see our figs. 22, 23, 28, 30, antea). This does not seem to be a difference of such constancy as to be relies on for the discrimination of species. It is the sole character distinguishing Miller's M. commoda from his M. gorbyi, not to mention M. scitulus and M. parva, which may be younger stages of the latter species. Similarly such specimens as E 7631, E 7635, E 7638, and E 7644, each with 5 plates, cannot otherwise be distinguished from E 7636 with 6 plates; while E 7630 and E 7634 with 6 plates are otherwise the same as E 7633, E 7640, E 7642, E 7677, and E 16168, each with 5 plates.