tinent; and the number of books, monographs and memoirs which ought to be searched is really very great. Moreover this is not always sufficient to guard against error, which can only be completely avoided by personal inspection of the foreign species to which it appears most nearly allied.
It is in such work, when the differences between nearly allied forms, described under different names, have to be carefully weighed and estimated, that the value of the fourth and last test of a good species is appreciated; the comparative value of such species is still more felt by any one who is instituting a comparison between similar faunas in different countries; for a given species, or varieties of the same, may actually exist in both formations, but, owing to their passing under different names, the fact may be disguised, and thus the number of species common to both is greatly underrated. To take an example, there are three species of Pecten to be found in the Cambridge Greensand, viz.:—P. orbicularis, Sow.; P. Barretti, Seeley; P. Raulinianus, D'Orb. Now those recently quoted from the Gaize of the Ardennes and Meuse are:—P. laminosus, Mant.; P. Dutemplei, D'Orb.; P. Raulinianus, D'Orb.; P. asper, Lam.; P. hispidus, Goldf.; and P. Galliennei, D'Orb. At first sight there appears to be only one species common to the two formations, viz. P. Raulinianus; but when it is known that P. laminosus is only a variety of P. orbicularis, and P. Barretti of P. Dutemplei, it is seen that all the Cambridge forms are represented in France.
It may, of course, be disputed that the two species are severally merely varieties of the two others; and I will admit that it is to a great extent a matter of opinion; but there can be no doubt that the species are respectively very closely allied, while with some of the others, such as P. asper and P. Raulinianus, they hardly possess any specific character in common; it is therefore very misleading that P. Barretti and P. Dutemplei, for instance, should be kept as distinct as if their differences were as great as those between the latter and P. asper. The only solution of this difficulty is, I believe, to be found in the more extended use of varietal names, and consequently of a trinomial nomenclature ; the form would then be expressed as Pecten Dutemplei, var. Barretti.
The new species subsequently described are:—
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Turrilites nobilis.
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Lima interlineata.
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Viewed by the criterion of the conditions above mentioned, it will be found that the Natica is of the most palæontological value, since several specimens have been found both at Cambridge and Folkestone; the Lima, of which three specimens have been examined, comes next in importance; the Nerita is only described from a single imperfect specimen, but it is a remarkable species and belongs to a genus that is rare in Cretaceous beds; the Turrilites is of least value, since it is founded upon a cast, and I have, indeed, had grave doubts whether it was worthy of being placed on record. The
