Methods 
Microstructural increments can be studied in thin sections of shell material, 
in acetate peel replicas of acid-etched shell sections, or under the scanning 
electron microscope (fractured or polished and etched shell sections). 
Acetate peels are the easiest and most rapid method of preparation for 
examination of most molluscan shells. The basic method of preparation, as 
outlined by Rhoads and Pannella (32), is as follows: 
Shells are embedded in a block of epoxy resin (e.g., Epon 815 resin with 
DTA hardener, 10:1 ratio, under vacuum; Miller-Stephenson Chemical 
Company, Danbury, Connecticut) to avoid shell fracture during sectioning. The 
plane of the cross-section passes from the umbo to the shell edge along the axis 
of maximum growth (30, 32). This cut is oriented so that growth increments 
intersect the plane of the section at right angles. The cut shell surface is 
polished sequentially with 350, 600, and, finally, 2600 or 3000 grade 
carborundum grits. The polished surface is then etched with 0.1 N HC1 for 
periods varying from a few seconds to a few minutes. Optimal etching time is 
related to shell structure, mineralogy, organic content, and state of 
preservation. It is recommended that a series of test etching times be carried 
out to determine optimum etching periods for a particular set of specimens. 
Etched shell surfaces are flooded with acetone, and a piece of sheet acetate 
is applied to the etched shell surface and weighted to avoid bubble formation. 
After the acetone (solvent) has evaporated (approximately 30 minutes), the 
acetate is removed from the shell and examined under the microscope (or used 
as a negative by placing directly in a photographic enlarger and printing). This 
technique yields excellent results for most species. 
Thin-sections are necessary for the examination of growth increments which 
are not structurally discontinuous, but instead recognizable only by dark and 
light color bands (32). For example, the growth increment boundaries in the 
deep-water species, Nucula cancellata and Calyptogena ponderosa , are 
indistinct and recognizable only by color variations of the bands, each band 
consisting of one dark and one light layer. The initial procedure for making 
thin-sections is the same as that for preparation of acetate peels, however, after 
the cut shell surface has been polished, it is glued to a glass slide using epoxy 
resin. The majority of the embedded shell and remaining embedding material is 
cut away using a diamond rock cutting saw, and the new exposed surface is 
polished sequentially until a 0.03 mm thick section of material is left on the 
slide. A cover slip is glued onto the newly polished surface using epoxy resin, 
and the material examined using optical or polarizing microscopy. Thin 
sectioning of shells is difficult, because shells tend to fracture when sectioned 
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