A. B. Honcood — Molluscan Shell-layers. 407 



my preliminary note on aragonite in the Middle Lias has not 

 prematurely precipitated that of his earlier research upon the matter 

 from a more general point of view. 



All geologists will be grateful to Professor Cole for giving the world 

 at large so full and well-digested a summary of the chemical side of 

 the question, as well as the result of the tests advocated applied to the 

 shells within his immediate purview (at Dublin) from an extensive 

 range of the geological formations. 



The object of these remarks is not to "go one better" than the 

 paper alluded to, but to clarify some misconceptions of my own and 

 others and to set the question upon a firmer and sounder basis for 

 future research. 



By the term ' aragonite shells ' it would appear that not only have 

 we (1) the shell before fossilization, exhibiting typical aragonite 

 characteristics, but also (2) the presumed former occurrence of it 

 in a fossil shell when the fossil is found to-day as a cast, or (3) 

 replaced by a granular form of calcite. Lastly, we have (4) the 

 known occurrence of certain classes of shells now as aragonite shells 

 in a fossil state. 



(1) Dr. Sorby ' gave a full summary of recent characters of the 

 shell-wall of recent shells, and it has been assumed that this applies 

 to fossil shells as well (though forgotten, as we shall see). The 

 classes he recognized as respectively exhibiting the specific gravity of 

 calcite or aragonite were thus summarized in a paper by Professor 

 P. P. Kendall. 2 They are contrasted here with a table given later — 



Calcite. Aragonite. 



Foraminifera. Corals (but Alcyonaria partly, Hydrozoa 



Annelids. mainly aragonite). 



Echinoderms. Cephalopods. 



Polyzoa (containing also some Gasteropods (except Patella, Fusus, 



aragonite). Littorina, Purpura, etc.). 



Brachiopoda. Conchifera (except Ostrea, Pecten, and 



Ostrea and Pecten. outer layer of Spondylus, Pinna, 



Cirripedia and all other Crustacea Mytilus). 



(the former exhibiting also some 



aragonite and phosphate of lime, 



the latter some phosphate) . 



(2) Sorby alluded to the occurrence of aragonite shells as casts in 

 limestone. 3 But whereas limestones are more usually composed of 

 calcite than aragonite, there are more calcite shells than the latter. 

 In rocks of clastic origin, e.g. the Coralline Crag, casts occur commonly, 

 and the shells belong mainly to the groups whose shell, when the 

 animal was alive, was of aragonite, according to the above scheme. 

 Professor Kendall gave a full list of these. 4 In the paper on the 

 Middle Lias referred to 5 I gave a short list of a few genera which 

 very frequently occur as casts. It is hoped to extend this and other 



1 Presidential Address Geol. Soc, 1879, pp. 29-30, reprint. 



2 Geol. Mag., 1883, p. 497. 



3 Presidential Address Geol. Soc, 1879, p. 35. 



4 Geol. Mag., 1883, p. 498. 



5 Ibid., 1910, p. 175. 



