A. R. Norwood — Holluscan Shell-layers. 409 



And again he wrote (p. 49) "freshwater shells of the aragonite 

 type" in referring to the Wealden limestones, showing that lie had 

 in mind their first, not fossil condition. 



As to pre- Jurassic formations, he writes (p. 56) of the Carboniferous 

 Limestone of "fragments of what were probably aragonite organisms" 

 as "now quite crystalline", and "show no structure", and (p. 57) 

 as to the Devonian limestones "completely disintegrated aragonite 

 organisms ", and under Silurian limestones as to the Upper Ludlow 

 rock (p. 60) of "some portions of what were probably aragonite 

 shells", and "few or no fragments of organisms not originally 

 calcite", and in the case of Aymestry Limestone of ' aragonite shells ', 

 in connexion with Wenlock Limestone, of " recrystallized fragments 

 of aragonite shells ". Thus, as Professor Cole remarks, it is apparently 

 the case that in rocks of pre- Jurassic age we cannot expect shells 

 to consist of aragonite (Gkol. Mag., 1911, p. 49). 



This use of the terms as applied to minez'al states of shells not 

 identical with their original state is to be regretted, and I have 

 therefore {infra) drawn up a nomenclature which I hope will prevent 

 any confusion in the future. 



Professor Cole, in his Aids to Practical Geology, 1902, p. 201, 

 distinguishes shells as 'aragonite shells' and 'calcite shells'. That 

 he deserves the credit for introducing a clearer nomenclature for these 

 various stages is obvious from the attention he has drawn to my own 

 loose use of terms and the present attempt to improve on them. 

 I would say here, however, that the expression "preserved in 

 aragonite " (p. 76 of my paper) should be taken as correct, but 

 that for the words just above, "when preserved as fossils," should be 

 substituted "the recent shells are of", and lower down "if not 

 removed by decomposition". For the list given by Sorby deals with 

 the types of shell in living shells, and the comparison made with 

 fossil types will not hold good all round, since, as pointed out by 

 Dr. Sorby, Professor Cole, and others, the aragonite is often replaced 

 by calcite as a granular pseudomorph. This false parallel has probably 

 caused the artificial division into calcite and aragonite shells, apart 

 from (1) alteration in the shell-substance or (2) possibility of different 

 shell-layers in the same shells. 



That I had no real reason for confusing the two classes of shell- 

 structure present and past is illustrated by the passages (ibid., p. 174) 

 "to the parts of shells which have not been converted into calcite", 

 (ibid., p. 175) " all have the outer original aragonite layer removed, 

 and are preserved as casts", and (p. 177) " aragonite as the replacing 

 mineral", but here 'replaced' should be read for 'replacing'. So 

 much by way of correction of slipshod terms, which I am the less 

 unwilling to admit because I am not the sole offender ! 



(3) "When the shell neither exhibits an aragonite layer nor an 

 original calcite layer, and is not a mere cast, it may be preserved as 

 a shell with the original aragonite layer replaced by calcite of a 

 granular character. Rose, in 1858, suggested that*shells found as 

 casts were originally of aragonite, those still preserved of calcite. 

 Sorby further (ibid., p. 35) remarks on the transformation of aragonite 

 into calcite : " when the aragonite shells are still represented by 



