In my “Notes on Drepanaspis Gemündenensis, Schlüter,” published in the Geological Magazine for April, 1900 (pp. 153–159), I described a paired plate of a trapezoidal form occupying a position on each margin of the under surface of the head, a little behind the mouth, and to which I gave the name ‘sensory’ plate, as it is perforated by an opening which, if not orbital, evidently marks the place of an organ of sense of some sort. With this opening I also identified a depression seen in some specimens in a similar position on the dorsal aspect, and came to the conclusion that its position was marginal, being seen in some specimens on the ventral and in others on the dorsal surface of the head. I was also much puzzled to find that in some specimens this pit or depression seemed to have a floor, covered with tubercles like those of the rest of the surface, a condition which apparently militated very decisively against the idea of its being an orbit.