207 
of the VentriculidiE of the Chalk. 
species of Ventriculidse exhibit is based on a modification of the 
simple jo/azY. Unimportant as the question may^ at first sight, 
appear, it is in reality of much importance. Every illustration of 
the Law of Unity is, and ever must be, interesting to the student 
of natural history, and important as the surest indication that 
some point of truth has been attained. And without the guide 
of some simple unity of this nature, it would be quite impossible 
to make the structure of the different species of Ventriculidse 
understood by the inquirer. The multiplied dissections which I 
have made is a labour which few would have the inclination, and 
as few the opportunity, to undergo. 
In the present genus and section we shall meet with one spe- 
cies in which the form of the simple plait is found without any 
modification. In the present species, and in several others, the 
primitive plait is not so obvious ; but the comparison of several 
specimens, and especially, as is so often the case in the illustra- 
tion of a Law of Morphology, of particular instances assuming a 
somewhat abnormal character, will enable the careful observer 
to trace, even in these species, this primitive plait. The limits 
within which I am necessarily restricted prevent me from entering 
more fully into, and illustrating at greater length, this question, 
— which I certainly regard as one of very much interest and im- 
portance. In the present section of the genus Ventriculites j 
in which every depression of one side answers to an entirely or 
nearly corresponding depression on the other, the existence of 
the plait is less material to the understanding of the character 
of the wall of the pouch than where the fold assumes that com- 
plexity which it does in the section Complicati. I shall dwell 
only slightly upon the point, therefore, in describing individuals 
of the present section, leaving the fuller illustration of it to those 
specimens from which the illustrations of PI. XIII. figs. 13, 14 and 
15 are drawn, and which will I hope satisfy every careful and 
candid inquirer that the most apparently differing external forms 
may depend on the different modifications which the upper and 
lower fold of a simple plaited membrane may undergo. 
3. Ventriculites quincuncialis. PI. VII. (vol. xx.) fig. 7, and 
PI. XIII. fig. II. 
Membrane deeply folded, usually from base to margin, of nearly 
equal width the whole depth of the fold, and in regular quin- 
wall of the pouch, where the whole body is a single pouch, from the subdi- 
vision of the body itself into distinct lobes or branches as found in Bra- 
chiolites. It will presently be seen that the wall of these lobes and branches 
is marked by the same characters as the wall of the single-pouch forms now 
under more immediate consideration. 
