209 
of the Ventriculidse of the Chalk. 
as to the fold of the membrane. It will often be triLimphantly 
referred to as affording evidence of perforation. There may be no 
primd facie reason why the Yentriculid?e should not, like the re- 
cent Retepora, 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 famil)^ 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 pi*(‘sent 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 /he/ in this 
case, and at the same time the importance of ascertaining tliat 
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 carefully 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, both 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 necessai'ily, 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. 2. Vol. i. 
14 
