16 



and contraction to reside in these bundles, as Parkinson* and 

 Dr. Mantell have done, from a similar appearance, in respect to 

 the Ventriculida3. 



A glance at the following figure will satisfy any one that a 

 simple and plain membrane may be folded up in the most intri- 



cate way without in fact destroying its simplicity, and having the 

 only effect of packing a larger surface into a smaller compass. 

 The mode of folding may be either longitudinal, in which case 

 we shall have the " cylindrical fibres radiating from the basis to 

 the circumference " of Dr. Mantell ; 'or it may be transverse ; or 

 it may be more or less intermediate between the two, thus 

 causing those longitudinal ridges to appear to anastomose ; or it 

 may be so regularly intermediate as to give to the surface a 

 mammillated appearance. The appearance of the body may even 

 differ on the external and internal surfaces, inasmuch as the folds 

 may assume a different direction as they reach the respective 

 surfaces, as we see familiarly in a rhubarb leaf just burst from its 

 sheath. But none of this can alter the nature of the membrane 

 itself, or serve to establish a generic character. It is further 

 evident that if the folding assume the mammillated character, 

 that is, if a membrane of some thickness be folded in and out in 

 regular figures, a section across any part will, according as its 

 direction shall be, represent a series of apparent tubules or of 

 reticulations regularly disposed, as seen in PI. VII. figs. 1 to 4 ; 

 and again, that if any foreign substance fill up any of the super- 

 ficial depressions on this mammillated surface, that surface will 

 appear to be regularly perforated by tubules. Hence the figures 

 of Goldfuss and Roemer, which profess to give magnified views 

 of the exteriors of Ventriculidse. The following figure will illus- 

 trate this : a, a, is the membrane, which is folded in and out with 



Fi. 



&L 



the exactest regularity, each fold being of equal breadth and 

 * Organic Remains, ii. p. 145. 



