212 Mr. J. Alder on Montacuta ferruginosa. 
But to return to Montacuta ferruginosa. The mantle, which 
is open throughout the entire front and base of the shell, is 
closed posteriorly, forming a small excretory orifice, not produced 
into a siphon. The foot, as might be concluded from the much- 
elongated anterior portion of the shell, is very large and mus- 
cular ; there is a slight angle about half-way down in front, be- 
yond which it is rather narrower and tapers to a blunt point : 
the base is slightly undulating and grooved through its entire 
length, though it does not appear to spread out into a flat disc 
like that of Lepton squamosum: the hinder portion is abruptly 
truncated. 
After having kept my specimen for some days in sea-water, I 
found one morning that the bottom of the glass was covered with 
a minute white dust, which I immediately concluded would be 
the spawn, and on placing a small portion under the microscope 
I found that such was the case. I consequently had it removed 
into a separate glass with a fresh supply of water, in order to 
observe its development. Though nearly round at first, the ova 
soon assumed a subtriangular shape, and about the third day, 
strong cilia were observed on one of the sides, and they began to 
rotate very quickly. One after another assumed the rotatory 
state, till nearly the whole were in motion. After rotating for 
about a day, they apparently burst the envelope, and swam freely 
about in the water in all directions, by means of their vibratile 
cilia, and at the same time assumed more or less of a bell-shape ; 
a slender style or thread projecting from the centre of the ciliated 
base. This organ, which has been observed in the embryos of 
other species, has been described as a kind of byssus, by which 
the little creature can fix itself securely to other bodies. This, 
however, I did not observe to be the case in the present instance. 
It soon appeared to be absorbed; the animal became gradually 
elongated, and the cilia were withdrawn into the shell, which 
then began to appear, but at what time it was actually formed I 
could not make out, as, from its extreme transparency and simi- 
larity of colour to the rest of the animal, it was very difficult of 
detection. The cilia could be seen vibrating within the shell for 
some time after the animal became quiescent ; a few isolated cilia 
at one of the extremities, not observed before, being the only 
ones that remained to perform their functions externally. These 
produced a partial current without propelling the animal through 
the water, as at this stage it gave up its natatory habits and took 
to a quiet life. The internal portion, the parts of which could 
not be very distinctly made out, appeared to be undergoing a 
process of development. The mass was continually changing its 
form, the separate parts being extended alternately in different 
directions, and a portion, probably the incipient foot, was occa- 
sionally pushed beyond the margin of the shell. At this point 
