43 
of the Ventriculidse of the Chalk. 
of difference. The inquirer is thus further relieved from the 
detail of specific differences by the division of each genus into 
sections. The still subordinate but constant points of difference 
last named will be characteristic of species. 
I have already alluded to the important and valuable test of 
the soundness of these principles of classification afforded, un- 
expectedly and after the work was completed, by the stratigra- 
phical harmony exhibited by the table of classification. It will 
be sufficiently obvious that the ocean of different ages would 
have such modifications as would not be adapted equally to all 
varieties. We accordingly find among the Ventriculidse, as in 
other divisions of palseontology, a few species enduring through 
many changes ; others dying out ; while with every fresh sera 
fresh forms display themselves. 
It will be understood from this, that mere size does not enter 
as an element into the determination of genus or species. Of 
many species I have specimens from an inch to eight or nine 
inches in diameter. It is not necessary to enter very fully, there- 
fore, into the question of growth. That question, always a difficult 
one in palaeontology, is difficult even in recent forms of the families 
allied to the Ventriculidae. It would be vain to hope to throw 
much light upon it by fossil forms. Where constant differences 
are found under all varieties of size, we are bound to consider 
them as distinct species. I shall touch briefly on the question 
of growth in introducing each separate genus. 
It will be also understood that the mere external (outward or 
inward) general form of the fossil does not enter as an element 
into the determination of genus or species. I have shown how 
deceptive that criterion must ever be. In the present instance 
the same general external form conceals essential differences in 
the mode and degree of folding of the membrane. 
It will occur to the reader that to follow the fold of a mem- 
brane, the trace of which is preserved only in a hard and solid 
matrix, must be a work of great difficulty ; and especially when 
that matrix is either so friable as the chalk, or so impracticable as 
the flint. The actual amount of the difficulty* cannot however be 
fully appreciated without actual experiment. The presence of that 
very oxide of iron, without which the forms could not be, in general, 
* In order that the actual nature, importance, and results of the present 
investigation should be properly understood, it is necessary to remind the 
reader that from the time of Dr. Mantell’s first work to his latest, and 
either by him or the other latest writers (see Portlock’s ‘ Report, &c.' p. 342), 
it has never been suggested or suspected that any membrane whatever existed 
in any of the Ventriculidse. They all describe them as composed of ana- 
stomosing “ cylindrical fibres,” (see ante, vol.xx. p. 76,) between which, on 
the inside, papillje or tubuli arise. I have demonstrated that the basis of 
the Ventriculidae is a simple unperforated membrane; that, therefore, the 
