of the Ventriculidse of the Chalk, 209 



as to the fold of the membrane. It will often be triumphantly 

 referred to as affording evidence of perforation. There may be no 

 prima facie reason why the Ventriculidse should not, like the re- 

 cent Retejmra, have been perforated. When however it is found, 

 through a vast number of differing forms, that one plan, that of 

 the folded membrane, has been unquestionably adopted; and 

 when it is found that in no other case in which Ventriculitic 

 structure is present is a thickness of more than five of the solid 

 squares attained by any part of the fold which forms the wall of 

 the pouch, while it is found that, in this species, the thickness of 

 the entire wall varies from one line to three lines, — thus exhibit- 

 ing, if the actual wall be but a perforated mass, the extraordinary 

 and, upon every principle, anomalous fact of there being no con- 

 stancy^ in the structure of the central polypidom ; there is cer- 

 tainly every reason to conclude that Unity is not violated in this 

 case, and that, like others of the family, this is truly a folded and 

 not a perforated membrane. The various deceptive appearances 

 which the mere fossil may put on have been already more than 

 once remarked. In the examination of the present species, 

 above all, the greatest skill and care are necessary, because it is 

 obvious that the very depth and narrowness of the fold must, if 

 any abrasion or injury of the external surface t take place, de- 

 stroy the traces of the membrane covering the top of each 

 depression, and because the small size of the depressions and 

 elevations renders it very difficult to follow the fold which forms 

 them from one surface to the other. Feeling therefore to the 

 fullest extent the difficulty of ascertaining the actual /«c^ in this 

 case, and at the same time the importance of ascertaining that 

 fact, both as an exception, if it should prove such, to the Law of 

 Unity, and as a determination of a point in palaeontology which 

 has been now mooted for more than thirty years, I carefi'Uy dis- 

 sected numerous specimens, both in flint and in chalk, and both 

 with the slitting- wheel and the needle, and followed up and down 

 the so-called (sometimes) cells or (sometimes) perforations to their 

 terminations. The clear and unequivocal result has been that the 

 so-called Ocellaria is no anomaly : that the Law of Unity has not 

 its exception here ; that this is a true case of fold of membrane : 

 finally, that I have at this moment before me specimens in which 



* It will be obvious to the careful inquirer that there is nothing anoma- 

 lous in the varying depth of the folds, inasmuch as, in all species, that depth 

 will naturally be dependent, more or less, on the age of the individual. 



t In every instance, botli in chalk and flint, with very rare exceptions, 

 some of the body adheres to the matrix. In the present species, where the 

 bases of the folds are so small, it will necessarily, therefore, often happen 

 that just those bases adhere to the matrix. Hence alone will result an ap- 

 parent perforation of the wall, the remains of the membrane not being pre- 

 sent, and not therefore traceable, over those bases. 



Ann. ^ Mag. N. Hist. Ser. 3. VoLi. 14 



