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SUPPLEMENT TO THE CRAG MOLLUSCA. 



life will he acquired, yet every increase of our knowledge of natural history must bring us 

 nearer to this state of things, and proportionately augment our difficulties in the way of 

 specific separation. In the present state of our knowledge, therefore, it seems to me more 

 philosophical, and likely to be more advantageous in working out the history of the past 

 changes of land and water on the globe, if the identification of species be not strained ; 

 and that, wherever a form presents differences from any other, though they be but slight, 

 and those differences are fairly maintained in a group of individuals without intermediate 

 forms occurring coevally in the same geographical area, such form should be regarded as 

 a distinct species ; in my ' Monograph of the Eocene Bivalves ' I have expressed my 

 views on that point more fully. Another reason for not undervaluing even slight 

 differences by which many of the Crag Mollusca are separable from their living analogues, 

 and so reducing them to the inferior importance supposed to be possessed by the term 

 "variety," exists in the discordance between the evidence presented, by the Molluscan 

 fauna when thus reduced, and that presented by the other organisms of the Crag 

 period. Thus the evidence of the Entornostraca, the Foraminifera, the Polyzoa, the 

 Polyparia, the Cirripedia, and the Echinodermata of the Crag (all of them studied and 

 described by independent authors) has quite a different bearing from that of the 

 Molluscan fauna, when this last is reduced in the way it has been by the author of the 

 ' British Conchology.' 



In the case of the Entornostraca the proportion of species not recognised as living is 

 as 13 to 5, of the Foraminifera as 47 to 53, of the Folyzoa as 65 to 30, of the 

 Polyjoaria 3 to 1, of the Cirripedia as 4 to 6, and of the Echinodermata as 13 

 to 3. 1 



Now, although the researches which have been carried on among the living species of 

 these groups of organisms may not have been so extensive as those carried on among the 

 Mollusca, and although we may thus be better acquainted with the living forms of the 

 latter, still, after making very large allowances on this account, we are left with great 

 discrepancies between the evidence afforded by the percentage of forms not known living 

 among the Mollusca, and those among the other groups. These discrepancies are so 

 striking as to suggest caution in accepting the process by which the list of Crag species 

 has been pared down, and so many species eliminated from it in the lists which accompany 

 Mr. Prestwich's Crag papers. 



The authors of the ' British Mollusca,' like myself, regard the Molluscan fauna of the 

 Coralline Crag as having its affinities chiefly with that of the Mediterranean ; but the 



1 A table of the proportions borne of living to extinct forms among these various groups of organism 

 will be found at p. 134 of Mr. Prestwich's paper on the "Cor. Crag." This agrees substantially with the 

 analysis given by me in the text. The number of species of Coralline Crag Mollusca, however, according 

 to the list by Mr. Jeffreys, which accompanies the paper of Mr. Prestwich, is 31G, of which he considers 

 264 to be living, and 52 extinct; thus giving a percentage of 84 recent and IG extinct. ' Quart. Journ. 

 Qeol. Soc.,' vol. xxvii, p. 128. 



