67 



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 Ventriculidae 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 F. latlplicatm 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 cannot doubt that, under similar circumstances of structure in ad- 

 joining parts, such 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 exhibited in the fossil condition, the 40th of an inch in diameter, a 

 space which the iron stain in the chalk is more than sufficient to obliterate. 



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