4-08 A. R. Horwood — Molliiscctn Shell-layers. 



details as to the nature of the shell-layer when preserved in specific 

 instances in a later communication. The causes assigned for the 

 occurrence of shells once possessing an aragonite shell-layer 1 are 

 recognized hy me (ibid., pp. 175, 177) as due to the causes assigned 

 to their decomposition in other beds by Professor Kendall 3 — («) enclosure 

 in permeable beds, (b) flow of carbonated water. The latter cause 

 was not alluded to directly by me, but is really an adjunct or necessaiy 

 outcome of {a). Further important remarks on the occurrence of 

 shells once possessing an aragonite shell-layer are given by Professor 

 Kendall and Mr. Cornish in the valuable paper quoted (antea). 

 Apparently the term ' aragonite shells ' was used by these authors 

 in a wide sense in this paper, for the existence of shells with an 

 aragonite layer in the Coralline Crag is alluded to, and this term 

 probably in the first paper (Geol. Mag., 1883, p. 497) did duty for both. 

 I must plead guilty in my own previous paper to a similar loose 

 terminology (to be corrected infra). That Professor Kendall used the 

 term without applying any other distinction than the prefix ' actual ' 

 to those still retaining the aragonite test alongside of shells without 

 any present shell-layer (in the rocks) is clear. Possibly the use of the 

 term has been due to the sanction of the term by Sorby, who uses it 

 for recent shells and rather often in his address (Geol. Soc), whilst he 

 also recognizes the occurrence of these same groups as casts in the 

 rocks and also with the aragonite now converted into calcite. 



Moreover, it appears that by the use of the terms ' aragonite Mollusca ' 

 and 'aragonite shells' in Lias (p. 54), in Great Oolite (p. 53), and in 

 Cornbrash (p. 51) and Cleveland. Ironstone (p. 54), ' calcite organisms' 

 in Oolites (p. 53), 'calcite Mollusca' (p. 52), 'calcite Polyzoa ' in 

 Portland Oolite (p. 50), ' aragonite shells ' in Purbeck Limestone 

 (p. 49) and Chalk (p. 48), 'calcite shells' in Kentish Kag (p. 49), 

 Sorby intended to convey the idea that they belonged to a type 

 in which to-day the shell is characterized by being composed of either 

 calcite or aragonite. In these passages he says nothing as to their 

 mineral structure at the period cited, which he examined. It appears, 

 indeed, that it has been left to later observers to point out what shells 

 still retain the aragonite layer. But in the following passages the 

 terms are used with accompanying indications of their state at the 

 time of examination (he of course makes general statements as to 

 the state of the calcite or aragonite in the fossil state compared 

 with its original structure), viz., under Tertiary limestones he writes : 

 "Being aragonite shells they have usually been removed or changed 

 into crystals of calcite," and as to the Barton Clay, "that of the 

 various Gastropoda, composed of aragonite, is as perfect as in living 

 shells." This surely means that Sorby had in his mind Barton Clay 

 Gasteropods, in which the shell was aragonite originally when the 

 animal was alive, and when he examined it also, or unaltered. 

 Surely Professor Cole cannot hesitate to accept this, though he says 

 it is not clear that Sorby meant to assert this. I think this is the 

 clearest passage in which Sorby indicates that the shells he examined 

 were still aragonite. 



1 As we shall see, this is the correct way to describe aragonite shells. 



2 Geol. Mag., 1888, p. 68. 



