and the micro-growth lines are obscured. In addition, it is difficult to avoid the 
formation of bubbles beneath the coverslip which obscure features. Equipment 
for preparing thin sections is available in rock preparation laboratories found in 
most geology departments. Alternatively, there are commercial firms (e.g., 
Rudolf von Huene, Pasadena, California) that will prepare thin sectioned 
material. 
Ecological Applications 
Several workers (9, 14, 18, 30-32) have suggested that information about 
physiological and environmental conditions may be recorded and stored in 
molluscan shells. Various studies in which workers have used microstructural 
increments within shells to extract such information are discussed below under 
appropriate sub-headings. 
Seasonal Cycles 
Seasonally caused annual growth rates and patterns are observable in all 
bivalves collected in climatic zones, ranging from cold-temperate to 
sub-tropical. In many species, as winter approaches, there is a gradual slowing 
down of the deposition rate, and the microstructural increments become 
gradually thinner. This slowing down of growth in the autumn culminates in a 
marked depositional break at the time of the first freeze. These depositional 
breaks are characterized by indentations of the outer shell layer, a dark band of 
organic-rich shell material extending downward from the base of the 
indentation, small daily growth increments on either side of the break, and a 
change in the shell structure near the break (9). These winter breaks may not 
be as marked in specimens living subtidally (32), although Mercenaria 
mercenaria from water depths of eight meters clearly show a distinct winter 
break (9). 
Through careful examination of microstructural patterns, Farrow (13) 
found that part of a population of the shallow sub tidal cockle, Cerastoderma 
edule, from the Thames estuary in England, stopped growing during winter due 
to sub-zero temperatures. Tevesz (34) observed that Gemma gemma grew very 
little in the winter. Growth increments were very closely spaced and the inner 
shell layer had a brownish hue. During the summer, G. gemma grew rapidly; 
microstructural increments were widely spaced, and the inner shell layer was 
clear and translucent in appearance. 
Through an examination of numerous acetate peels, Evans and LeMessurier 
(12) were able to demonstrate striking winter growth rate differences between 
two sympatric species of bivalves. They found winter growth of the 
rock-boring clam, Penitella penita, to be approximately 75 percent of the 
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