of the Ventriculidse of the Chalk. 207 



species of Ventriculidse exhibit is based on a modification of the 

 simple plait. 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, 

 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. 11. 



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. 



