214 MR. W. ARCHER ON THE MINUTE STRUCTURE AND 
convex surface being towards the cavity of the cell. Strictly speaking, that portion 
of the superficies had better be called truncate rather than plane, as, especially in 
the larger examples of these bodies, it сап readily be seen to be hollowed out or ex- 
cavated in a rather irregular manner (Pl. XXVIII. figs. 11, 16, 18). Sometimes this 
hollowing out is a mere simple concavity ; or this may show at its middle а secondary 
depression, or even a tertiary somewhere about the middle of the secondary, with some- 
times a narrow inlet, then roundly expanding within. Thus the more or less irregular 
excavation may sometimes reach to a depth of a half to three quarters of the thickness of 
the whole body, but more frequently to only a quarter to a third the depth. Sometimes 
the excavation is nearly vertical laterally, and its bottom flat, in place of forming a concave 
depression, whilst that of an interior median secondary depression may be rounded or 
flat; and sometimes (though rarely) a narrow perforation may even pass right through to 
the hemispherical surface. In the larger examples especially, the inner surface of these 
inwardly variously shaped, though usually at their periphery pretty nearly circularly 
bounded excavations is seen to be not smooth but rough, even rugged, and of a somewhat 
granulated aspect. 
When the two knob-like covers or stoppers are undisturbed in situ—that is to say, 
placed directly opposite to one another, each upon the opening of its own pit, and thus 
closing it up—and when the two together are viewed in conjunction with the outline of 
the two pits forming, as it were, a canal slightly narrowing halfway from one knob to the 
other, the whole offers a very curious and singular resemblance to a “rivet,” as it were 
binding together the neighbouring joints; for the canal of the double pit is sufficiently 
deceptively like the shank of the quasi-rivet, the variously figured stoppers (as described) 
passing for its “ heads,” which clamp the structure on both sides. But when, as often 
happens, the stoppers become dislocated (see woodcut, р. 230), the quasi-stalk (that is the 
double pit) of course remains, dispelling an illusion of which the observer might at first 
glance be pardonably enough the victim. 
These stoppers did not offer any evident reaction in presence of the usual tests for 
starch and cellulose; they disappeared totally on the specimen being boiled in caustic 
potash. 
They are, so to speak, external to the so-called * primordial utricle," yet in intimate 
contact or union therewith. One could occasionally see the cell-contents still closely 
adherent to the convex surface of a stopper, partially contracted, and stretched into а 
conical shape, more and more expanding in width away from the stopper. Oftentimes the 
tension so induced caused the dislocation and pulling away of the stopper still coherent 
with the mass of plasma; or it might be “ let go” halfway ; at other times, and indeed 
more frequently, the cell-contents contracted, leaving the stopper i» situ. In otherwise 
seemingly empty cells the stoppers, if not in their places (see woodcut), as is very fre- 
quently the case, could be found lying about in any part of the cavity and in any post 
tion. By their comparative dimensions one might safely enough predicate the particular 
pits to which they doubtless had belonged, and whence they had become dislocated 
(Pl. XXVIIL fig. 3). — 
But on examining the organization of this pretty alga, and in trying to gain a know 
