of the same intrinsic value as those which are founded upon a number of perfect examples; neither can they be of the same use or importance to the comparative palæontologist. We are led, therefore, directly to the conclusion that some species are of more palæontological value than others ; and it may be worth while considering a little more fully the other circumstances by which this value is modified or increased, especially as I am not aware that any one has recently discussed the conditions under which new species may be constituted, or given any directions to those intending to name and describe such forms as may appear to have been previously unnoticed.
The importance of a proposed new species must greatly depend, I think, upon the following circumstances:—
1. The number of specimens examined (whether many or few).
2. The number of localities where it has been found.
3. The extent to which the original shell or test has been preserved.
4. The amount of difference between it and nearly allied species of the same genus.
In the first place it is evident that a species founded on a single specimen, however well preserved, has not the same value as one the description of which has been drawn from an examination of many individuals. Much, however, will depend upon the character of the fossil; and supposing all the other conditions to be very favourable, if it is in a good state of preservation, retaining all its parts, and if it be markedly different from any other species, so that it is unlikely to be merely a monstrosity or local variety, then its intrinsic value is much enhanced; but at the same time it remains of little use when considered as a member of the fauna to which it is added; for, being found only at one locality, it cannot be utilized in a comparison of one fauna with another, but must always be omitted from such calculations.
Again, new species which have been described, from single imperfect specimens in the state of casts, and even those founded on casts which are abundant and perfect so far as they go, are, as a rule, very unsatisfactory; for it is only in exceptional cases that the cast indicates the form and ornamentation of the shell with which it was covered. In cases where the cast does give such evidence its description is perhaps of some value; but where the casts are nearly smooth while the test was probably ornamented, as in Bivalve shells, Echinoderms, and many Gasteropods, it is worse than useless to describe them; for it is often impossible to compare them properly with specimens from other deposits which retain the shell, and the result is an unnecessary multiplication of specific terms: it is far easier to give a specific name to such casts than it is to disprove their title to the same.
Again, it is exceedingly difficult, even where specimens are abundant and their state of preservation good, to be quite sure that the form has never been elsewhere described. If not known in the British Isles, it may have been figured and described on the Con-
