A. R. Horwood — Molluscan Sliell-layerx. 



417 



In order to give some notion of the variability of the structure 

 of the shell-layer in a typical collection of molluscan fossil shells, 

 I have endeavoured to indicate above (pp. 412-17) the characters 

 of the species represented in the palseontological collections 

 in the Invertebrate Department of Leicester Museum arranged 

 biologically. 



As this collection exhibits allied genera in proximity, it serves better 

 for this purpose, which is essentially a biological investigation, than 

 a study of the same material arranged stratigraphically. I might 

 have undertaken to examine the shell-layers in the stratigraphical 

 collection, but as its arrangement lias only just been completed pro tern., 

 and it requires to make it complete a good deal more material, 1 

 I have left that till later. The following list, however, contains 

 a representative series of genera, and in some cases species identical 

 with those enumerated by Professor Cole, with varying results. The 

 few cases in which aragonite still remains unaltered are sufficiently 

 rare to warrant the assumption that its preservation is exceptional in 

 Jurassic time, and though of more frequent occurrence in some places 

 in Tertiary time is still less common in the intervening Cretaceous 

 period. 



1 Any geologist who has duplicates may contribute towards this desideratum 

 if he or she will. 



DECADE V. — VOL. VUI.— NO. IX. 



27 



