48 CRETACEOUS GASTROPODA 
colours of the different genera, is of the highest importance in the determination of 
fossil species, because when it is removed the shell receives a totally different aspect. 
The variety of the striation and tuberculation in Cyprea, Ovula, Trivia and others 
belongs to that enamel coating and usually disappears with this altogether, leaving 
no trace of its previous existence. The same applies to the dentition of the margins 
of the aperture. 
We may meet with descriptions of crprazvz in three different stages: 
i. Being in a perfect state of preservation, in which case there cannot be any 
difficulty in determining the genus properly. Up to the present time, we may say, not 
a single cretaceous specimen has been found in this desirable state. The most com- 
plete is a specimen of Cyprea Newboldi, of which a representation is given (Fig. 2.) on 
our Pl. IV, but even this has the enamel surface a little injured, and the dentition 
of the lips has been partially lost in exposing the surface from the adhering rock. 
ii. In recent species of crpr#ivx the enamel coating is very closely combined 
with the shell, and a separation of both offers no small difficulty. It is known that 
the enamel has been separated by the mantle in successive layers, and that its 
thickness differs as well in different species, as on different places upon, and in 
different stages of age of, one and the same shell. When a Oyprea has been 
exposed for a long time to the changes of the climate, and when the shell has 
often been acted on by different mineral solutions, it is observed that the layers 
of the enamel begin to separate and fall off partially or totally. This may happen 
sometimes on the shores of the sea itself, before the specimens are finally buried 
in the rock; not uncommonly also the enamel may adhere to this more strongly 
than to the shell, and the latter be deprived of it in being cleared from the rock. 
In all cases such apparently perfect specimens with the shell—exhibiting usually 
strie of growth—must be always very cautiously examined. They do not show 
eenerally the slightest trace of dentition on the margins of the aperture, and still 
appear to be in every way perfect. Iam inclined to think that this state of pre- 
servation has especially led D’Orbigny and other authors into all those misappre- 
hensions about Ovula and Cyprea. The enamel is often so uniform with the 
calcareous rock in colour and texture, that there is actually an impossibility of 
separating both. I have probably examined a larger number of cretaceous 
crpraéin# than any other paleeontologist, and I know how many hours are often 
lost in their preparation without any success. A drop of acid does sometimes more 
than every other attempt; although obliterating the greater part of the surface, it 
still often leaves some traces of dentition perceptible, if any were present. In prepar- 
ation with the needle alone, they are often lost without a perceptible trace. 
iii. The shell may be perfectly destroyed, and this is, among the known cretace- 
ous species, by far the most common state of occurrence. The species of Cyprea 
and Aricia show then generally an elevated spire, while those of Luponia, Trivia and 
others are usually quite involute. We need not be surprised, that several concho- 
logists insisted upon not acknowledging any species, based on casts of the shells only. 
In some cases casts of shells may no doubt be very instructive, and if properly 
