Many characteristics of bivalve shells are related to environmental rather than hereditary conditions. If these acquired characteristics could be recognized and interpreted in fossil shells they would be of considerable significance in paleoecology. In this study a number of growth experiments were conducted on living representatives of the Family Pectinidae, a group of bivalves with an important fossil record. These experiments established the presence of several kinds of acquired characteristics in pectinids. The fine concentric ridges present on many pectinid shells are shown to be daily growth lines, formed in response to a biological rhythm regulated by the cycle of light and darkness. Daily growth increments are also seen in radial sections through pectinid shells. These lines can be used as time markers to greatly increase the amount of information which can be derived from other acquired characteristics. For example, variations in growth rate are known to result from variations in environmental conditions, and these can be observed and measured by means of the daily growth lines. Moreover, they can be correlated between animals grown under the same conditions; this offers a technique for determining the temporal relationships between specimens in a fossil assemblage, a problem of some concern in paleoecology. Daily growth lines were used to relate variations in shell chemistry (magnesium concentration) to time of growth, and thereby to variations in growth temperature. There appears to be a great potential for applications of this kind. Experiments on Leptopecten latiauratus latiauratus and Leptopecten latiauratus monotimeris, two very different pectinid subspecies, strongly suggest that the morphological differences between the two are principally due to environmental differences. This has important applications for the paleoecology of these forms, and suggests that such strong environmental effects might not be uncommon in the fossil record.