56 



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. figs. 2 & 3*. 



Membrane 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- 

 tra, be found to this general rule, both external and internal 

 surfaces of the Ventriculidse were polypiferous. And notwith- 



* The object of fig. 2 is to illustrate the very deceptive appearances which 

 the difference in the state of perfection of the fossils Mill occa^on. 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. 



