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How the Oyster makes his Shell.—Professor Samuel Lockwood, in a recent lecture before the New York Microscopic Society, answered the question which is asked by the fool in "King Lear"—"Canst thou tell how an oyster makes his shell?" He starts with the hinge-end, at the spot known to conchologists as the umbo. "A small plate, or single scale, now represents each valve, and that is the first season's growth. The next season a new growth, or plate, shoots out from underneath the first one, just as shingles do. The oystermen call these laps, or plates, 'shoots,' and they claim that the number of shoots indicates the years of the oyster. They certainly do contain a record of the seasons, showing the slow-growing and the fast-growing seasons. ... I have likened these shoots to shingles. Now, at the gable of a house the shingles may be seen edgewise. So on the side of an oyster-shell is a series of lines. This is the edge-wise view of the shoots, or season-growths. Another factor is the purple spot, or scar, in the interior of the shell. It is the place of attachment of the adductor muscle. Its first place of attachment was close up to the hinge. Had it stayed there until the shell had become adult, how difficult would be the task of pulling the valves together!—the leverage to be overcome would be so great; for we must bear in mind the fact that at the hinge-end the valves are held by this black ligament, which is, in life, elastic, swelling when the shell opens, and being compressed when the animal draws the valves together. So, with every year's growth, or elongation of the shell, the mollusk moves the place of attachment of the muscle onward, that is, in advance farther from the hinge. As it does so, it covers up with white nacre all the scars that are back of the one in actual use as the point of attachment of the muscle." To make the similitude of the oyster's shoots, or season-growths, with the shingles on a roof complete, "it would be necessary for the bottom shingle on the roof to underlie the whole series, and reach even to the roof-tree, or ridge-pole. Then the second shingle from the gutter must in like manner underlie all the rest of the series; so of the third, and so on with the rest. In this way lie the shoots, or laps, of the oyster's shell. The last one deposited underlies them all, and every one terminates at the channel in the bill—so that this groove in the bill contains a series of transverse lines, each one marking a season, or a year. Thus we get really four factors for the solution of the question, 'How old is the oyster?' all of which are the outcome of the method or way of making the shell."
The Trap-Dike of Southeastern Pennsylvania.—Professor H. D. Rogers, in his report for 1858 on the geology of Pennsylvania, refers to two trap-dikes in the southeastern part of the State. In the map published in connection with Professor J. P. Lesley's survey, Mr. C. F. Hall connects the two dikes so as to make a single dike about eight miles long. Professor H. Carvill Lewis, after two years of observations, has found that this dike is only a small part of a long, narrow dike, which passes almost entirely across the southeastern part of
