NOTES ON THE CAMBRIDGE GREENSAND. 487 



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 com- 

 pletely 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. Baulinianus, D'Orb. Now those recently quoted from 

 the (raize of the Ardennes and Meuse are : — P. laminosus, Mant. ; 

 P. Dutemplei, D'Orb. ; P. Baulinianus, 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. Baulinianus ; 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. Baulinianus, 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 conse- 

 quently of a trinomial nomenclature ; the form would then be ex- 

 pressed as Pecten Dutemplei, var. Barretti. 



The new species subsequently described are : — 



Turrilites nobilis. Lima interlineata. 



Natica levistriata. Nerita nodulosa. 



Nautilus, sp. nov. 



Viewed by the criterion of the conditions above mentioned, it will 

 be found that the Natica is of the most paheontological value, since 

 several specimens have been found both at Cambridge and Folke- 

 stone ; 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 



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