A. R. Horicood — Molluscan Shell-layers. 



411 



conform to the usual type, e.g., Patella, Fusiis, Litiorina, Purpura 

 in Gasteropoda, Ostrea, Pecten, Chama, Spondylus, Pinna, Mytilus in 

 Laraellibranchiata. 



I am quite aware isolated instances (and even published figures, 

 e.g. Hyatt's) have been given of separate layers in fossil shells, but 

 tbe prevailing custom has been to treat them as one, and as calcite or 

 aragonite, never both. The very names 'aragonite shells', 'calcite 

 shells ' show the laxity into which we have fallen. The addition of 

 a nacreous layer in Lamellibranchs and Cephalopods is an argument in 

 favour of their derivation from Gasteropods, and not vice versa. 



Since there is some confusion in the use of the terms calcite and 

 aragonite shells applied to fossil shells, I have drawn up below a table 

 of terms and states which may be useful in avoiding the confusion 

 inevitable unless the present state and age are both stated parenthetically 

 with each separate use of either names, and as a name, even if it 

 is a bad one, is better than a parenthesis, we offer the following 

 terminology as a beginning in this direction, incorporating existing 

 ones that are easily intelligible. 



