216 Mr. Toulmin Smitli on the Classification 
from base to margin with slight occasional constrictions : 
inner plaits depressed at regular intervals ; bulging on each side 
around depressions till adjoining plaits meet and open into 
each other : processes conspicuous : pouch very often nearly 
cylindrical : wall thin. 
Marked specimens of these two varieties are very distinguish- 
able, but, upon the principles to which I have confined myself in 
discriminating species, I do not think it proper to separate them ; 
the elements of the modification of the fold being similar in each 
variety. Specimens in which the two varieties run into each 
other are not unfrequently found. 
The variety tenuiplicatus is a most beautiful and delicate fossil. 
I have several specimens on which the polyp-cells are finely seen. 
In examining each of these varieties the importance of the clue 
afforded by the principle already explained of Unity in the fold 
of the membrane becomes strongly felt. Let the reader compare 
figs. 13, 14 and 15 of PI. XIII., always remembering that fig. 13 
is the basis of every modification. In fig. 15 he will see the lower 
plait [it is immaterial for the present purpose whether that is to 
be looked at as the inner or outer surface] retaining its simpli- 
city, while along the top of the upper plait he will see a series of 
circular depressions at short and equal intervals. At the sides of 
these depressions the membrane bulges out, — in order to pre- 
serve the walls of the plait below the depression clear from con- 
tact*, — until the bulgings on the two adjoining plaits meet and 
anastomose. These then open into each other in the centre of 
the place of anastomosis, leaving, however, below them and on 
each side a clear space ; so that strength and security are gained 
to the whole mass, and, consequently, the safety of the individual 
* This is but one other among the numberless beautiful illustrations 
afforded by the anatomy of the Ventriculidse of obvious design and adap- 
tation. Did these walls touch, there would be a great loss of surface and 
little or no additional strength. By the actual arrangement there is a vast 
gain of both surface and strength, as well as an additional security for free 
access of sea-water to all parts of the inner surface. It has already been 
seen that this additional provision not being needed in V. latiplicatus is not 
found in that species. It is proper to add, that the fact that, around the de- 
pressions, the adjoining plaits do not only anastomose but open into each 
other has been ascertained by most careful examination and dissection of 
specimens in chalk and in flint ; but that, having clearly ascertained the fact 
in instances where there is thus such an obvious purpose for such an arrange- 
ment, I cajinot doubt that, under similar circumstances of structure in ad- 
joining parts, sucli is always the arrangement of the fold in this particular, 
though the minuteness of the parts generally prevents the possibility of 
clearly distinguishing it in chalk specimens. The place of the opening does 
not usually exceed, in the necessarily somewhat collapsed state in which 
it is exliibited in the fossil condition, the 40th of an inch in diameter, a 
space whicli the iron stain in the chalk is more than sufficient to obliterate. 
