410 A. R. Hor wood — Molluscan Shell- layers. 



a distinct calcareous layer, this has no longer the original structure, 

 hut is more or less coarsely crystalline, and quite unlike the con- 

 tiguous calcite shells which still retain their original microscopical 

 and optical characters." 



Professor Cole (ibid., p. 50) has referred to the passing of aragonite 

 into calcite in the fossil state, and the record of fossils he gives, where 

 the shell-layer is determined as calcite in the case of those originally 

 possessing an aragonite test, forms the nucleus of our specific enumera- 

 tion of such cases. In the table here appended additional instances 

 are given. 



(4) The occurrence of aragonite in the shell-layer of fossil shells 

 and other organisms is definitely remarked upon by Sorby (ibid., p. 47) 

 in regard to the Barton Clay. Messrs. Kendall and Vaughan Cornish 

 refer to actual aragonite shells (ibid., p. 68) in the Coralline Crag, 

 species of Fusus (p. 70), Cephalopoda from the Cretaceous, Belemnitella, 

 Artemis lentiformis, and in JEschara an inner calcite and an outer 

 aragonite layer. They state (p. 72) that "the Mollusca always have 

 the aragonite layer internal". But this is not so, for in Nautilus it 

 is external, as in all Ammonites. The aragonite nature of porcellanous 

 Foraminifera is referred to and their absence in certain formations 

 noted, though the other group Vitrea occurs, being explained by the 

 supposition that they have disappeared or been obliterated, being- 

 represented only as casts, which give no indication of their former 

 existence. 



In my paper on the occurrence of aragonite in the Middle Lias 

 I have given instances of aragonite shells still preserving their test 

 unaltered. Further, Professor Cole has collected cases of the 

 occurrence of aragonite shell-layers in fossil shells from Jurassic to 

 Recent times, and he admits the occurrence of these in the Lias, 

 adding (ibid., p. 55) that I "may be justified in expecting some 

 retention of aragonite even in Jurassic strata ", though in his opening 

 remarks he asks the question whether they are preserved (evidently 

 meaning now) in aragonite, a question he answers for himself, in other 

 instances affirmatively, en passant. 



So far we have considered the question, almost exclusively, as 

 though only one shell-layer {either calcite or aragonite) existed in 

 fossil shells, regardless of the admitted existence of more than one 

 layer in both Lamellibranchs and Cephalopods. And though inci- 

 dentally Messrs. Kendall and Cornish refer to two distinct layers in 

 one shell [ante), this is apparently the only gleam of light upon the 

 question in all previous research on the shell-layer of fossil molluscs 

 (I should not omit to say that Bather and Hyatt fully recognized this 

 in writings in reference to structure, apart from fossil preservation). 

 For previously everyone, though admitting (tacitly) the homology 

 between the structure of recent and fossil shells, has ignored the 

 triple or double character of the former in stating conclusions as to 

 the chemical composition of the latter. 



In the following table we give a summary of the nature of the 

 shell-layer in the different groups of Mollusca, living and fossil, 

 for comparison, omitting for present purposes those exceptions in both 

 Gasteropoda and Lamellibranchiata in which the shell-layers do not 



