205 
of the Ventriculidse of the Chalk. 
with a much smaller-meshed tissue which appears dotted all 
over, the dots being the polyp-cells. When the polyp-skin is 
present it hides each of these appearances, and is marked only 
with slight depressions in the places where the dots are seen 
under the conditions last named. 
The traces of the moveable processes appear slight, often quite 
wanting, in this species. This is what might be expected if the 
object of those processes was to sweep the surface clear of ob- 
structions from the mouths of the cells. The surface being plain, 
such obstructions would be far less liable to accumulate around it 
than near those rugose surfaces which other species present, and 
on the most rugose of which the processes are most strongly 
marked. 
I have specimens varying from 7 inches to 1 inch in dia- 
meter. 
It appears to me that the fossil called by Dr. Mantell Spongus 
Townshendi (South Downs, p. 164) is nothing more than a V. 
simplex^ in flint. 
2. Ventriculites impressus. PI. VIII. (vol. xx.) figs. 2 &3*. 
jMembrane more or less slightly folded at upper part, but gene- 
rally without regular figure : moveable processes frequently 
conspicuous. 
The basal portion of this species is usually plain like V. sim- 
plex, but it soon exhibits marks of fold. These are sometimes 
slight, sometimes deeper. They vary much in different speci- 
mens, so that it might be thought advisable to distinguish varie- 
ties. It has seemed to me, however, better to include all under 
one name only. They may be generally known at once by the 
basal portion being quite plain as in V. simplex ; by the much 
less depth of the folds than in the next species ; by the depres- 
sions being usually more or less oval instead of round ; and by 
the want of any regular figure in the arrangement of the folds. 
This being the first species in which the membrane is folded, 
it will be well to consider the principle, if any, upon which the 
fold in all the Ventriculidse is arranged; and also to inquire whe- 
ther the polyps existed, during the life of the animal, over the 
inner parts of all the folds. 
To treat of the latter question first : I have already intimated 
that, as a general rule, — exceptions may, as in Eschara and Flus- 
* The object of fig. 2 is to illustrate the very deceptive appearances which 
the difference in the state of perfection of the fossils will occasion. On the 
lower part of that figure all remains of the actual body are lost. On the 
upper part an Ostrea is seen, and, near it, a small part of the actual body 
remains. The difference between this and the rest of the figure must be 
very apparent. 
