. Bulletin. La science ; natural history ; histoire naturelle. IV7.1 SIIIM MACNI".ET SI'DÉPUTÉ IIIIM EN Garniture-1 J ', (>l I I IVI A. 0,24 cm Figure I. Vue en plan d'un shell Tivela typique de stultorum montrant la position sur chaque shell utilisé pour l'analyse. Méthodes d'échantillonnage.-Five Live de myes ont été recueillies auprès de chacun des deux plages de Californie séparés par 200 kilomètres de côtes-Pismo Beach et Torrance Beach. Tous les dix ont été recueillies les palourdes "in situ." Le p. 100 MgCO:l et SrCO :, ont été déterminés pour les différentes parties de chaque spécimen par des méthodes d'absorption atomique flamme spec

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. Bulletin. Science; Natural history; Natural history. IV7.1 MACNI'.SIIIM AND SI'HON IIIIM IN Sill-1 I ', (>l I IVI I A. 0 .2 4 cm Figure I. Plan view of a typical shell of Tivela stultorum showing the position on each shell used for analysis. ANALYTICAL METHODS Sampling method.—Five live clams were collected from each of two California beaches separated by 200 miles of coastline—Pismo Beach and Torrance Beach. All ten clams were collected "in situ." The percent MgCO:l and SrCO:, were determined for various parts of each specimen by standard methods of atomic absorption flame spectroscopy using the Perkin-Elmer Model 303 Atomic Absorption Spectrophotometer. From each locality two right valves and two left valves were analyzed. In order to determine accurately any possible trend of variation throughout the shell, the valves were sampled continuously in two perpendicular directions: a) the anteropos- terior plane crossing the muscle scars, and b) the plane perpendicular to the growth lines cutting the beak and continuing directly between the muscle scars (Fig. 1). The beak area and denti- tion were excluded from the analyses due to the complicated structures present in these areas. Thirteen positions were analyzed—seven down and seven across with the center position being common to both directions. On three valves four additional positions were analyzed. Each position was approximately % inches square and occupied the thickness of the valve at that particular posi- tion. Three clams were also analyzed for varia- tions within the thickness of the valve—a sample was cut parallel and equidistant to the interior and exterior planes, and the interior material was compared with the exterior material. Anal) es. I or each sample M performed per element, The pep' ion ol this technique was 13 percent foi magnesium a 17 percent for stronti The foui standard for magnesium in this experimenl were 0.2 ppm, 0.5 ppm, l.o ppm, and 1.5 ppm. Three standards were us

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