ony 
Miscellaneous. 137 
of a nearly regular, convex, conical form and nearly central tip. The 
animal of this shell has dissolved a space on the surface of the other 
Dispotea of the size of the edge of the aperture of its shell. 
No. 5. is a Cardita with a Dispotea on each of its valves placed 
as in specimen No. 4, that is, with one of the edges of the shell close 
on the lower edge of the bivalve; and there is a single valve of the 
same species of Cardita with another Dispotea in a similar situation. 
It is to be observed, that under each of these shells, instead of the 
animal having eaten, or rather dissolved away part of the surface of 
the bivalve soas to form a smoother surface, each of the animals has 
deposited on their supporter a circumscribed layer of rather transpa- 
rent hard calcareous matter of the exact size and form of the mouth 
of the shell, which fills up the greater part of the space between the 
ribs and forms an even and smooth base, and in one case it covers over 
some Serpule and other bodies which were attached to the bivalve. 
I cannot find any indication of a muscular scar on this deposit. 
These Dispotee have a thick pale brown shell, darker towards the 
upper part of the cavity ; the outer surface is covered with thick, ir- 
regular, radiating, flattish-topped ribs, crossed by irregular concentric 
ridges, having oblong or linear intervening nets, and the surface of 
one of the specimens is marked with some irregular cross-ridges 
caused by the inequalities of the shell. In one of the Dispotee the 
internal appendage or back of the shell is near the lower edge of the 
bivalve, and the other has it near the umbo. 
I believe that the. whole of these specimens. belong to a single 
species (No. 1 to 4 is D. tubifera, Say, and No. 5 is D. rugosa, Les- 
son), but it is curious to observe, that when within the cavity of an- 
other shell, it is white, low, and the animal did not dissolve any part 
of the surface to which it was attached; that when on the outer 
surface of the shell, it is high, thick, dark brown, and in some cases 
it absorbs the surface to which it is attached ; and at others that it 
deposits a layer on the surface of the shell to which it is affixed, of 
the size of the margin of the shell itself. I may observe that generic 
characters have been formed on less variations in habit and less 
characters. 
In the same collection are two specimens of Pecten with two Crepi- 
dula on each: they have modified the form of the surface of each shell, 
and the animals have absorbed a very thin layer from the surface of 
each part of the shell to which they are attached.—J. E. Gray. 
On the Embryology of Acton. By M. Voer*. 
The embryology of the Actgon has been the principal object of 
my researches; I have seen the coupling of this interesting little 
mollusk, I have been present at the laying of the eggs, which takes 
place during some hours after the coitus, and I have thus had an 
opportunity of following, from hour to hour, up to the present day, 
the changes which the egg undergoes during a month. I have thus 
been able to ascertain that the separation of the vitellus is complete 
in this species, and that the division into eight parts offers a very 
* Extract of a letter addressed to M. Milne Edwards. 
Ann. & Mag. N. Hist. Vol. xvii. L 
