o/ Ventriculidije o/ Me Chalk. 217 
polyps is ensured, together with a great increase of polypiferous 
surface. 
I am aware of no instance among the Ventriculidse in which 
a mere ridge or flap of membrane is added on, — as this 
would practically be if the membrane forming the sides of these 
depressions came in contact with that forming the sides of the 
plaits. There are, on the other hand, very many cases in which, 
as in the present species, additional strength to the whole mass 
is afibrded by a means which gives a very great additional surface 
for the development and security of that life, the manifestation 
and multiplied means of enjoyment of which were the end of the 
existence of the whole creature. 
On each side of the places where these meeting bulgings blend, 
the vacant spaces between the plaits put on, naturally, from the 
shortness and regularity of the distance between the bulgings, a 
circular form. As one of these must, of course, lie between each 
double pair of the depressions, a quincuncial figure is thus as- 
sumed by the whole (see the darker spots in fig. 15 of PI. XIII.). 
The inquirer will thus at once understand the appearance pre- 
sented by the inner surface of many species of Ventriculidse. He 
will clearly see that the tubuli of Dr. Mantell are things with- 
out existence. He will understand that though, at the surface of 
a fossil of which all the interstices are filled with chalk, all the 
circular marks appear alike, yet the dissection of alternate rows 
of them will reveal very different conditions ; one row consisting 
of more or less shallow (in the present species usually rather 
shallow) and closed depressions ; the other having no true de- 
pressions at all, but consisting of cavities extending to the bottom 
of the intermediate plait, and indeed, before and behind, under- 
neath the intermediate bulgings, running into and forming part 
of the longitudinal cavity of that plait. 
The wall of the variety tenuiplicatus being usually thin, the de- 
pressions on the inner plaits, though shallow, are sometimes to 
be seen through on the outside, if the specimen has come very 
clean out of the chalk. It is very rarely, however, that specimens 
can be got out of the chalk in this clean manner, portions of the 
matrix usually filling up the spaces between the plaits. In the 
recent state, and when, instead of being in the collapsed condi- 
tion in which we find the fossils, the animals were alive and fully 
distended with all their fluids*, no doubt the lower surface of 
all the depressions on the top of the inner plaits could be seen 
from the outside. The living creature must have appeared as 
composed of a number of plaits which, on their inner surfaces. 
* Every reader m\ist perceive the difference between tlie comparative 
slates of collap.sio/i and distension and those of contraction and expansion. 
