Monographs of the United States Geological Survey (Volume 44)

 
9781235687679: Monographs of the United States Geological Survey (Volume 44)
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This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1903. Excerpt: ... PLACENTICERATIDE Hyatt. The young are similar to the adults of Engonoceratidae, having concave venters bordered usually by continuous ridges with compressed volutions, the lateral zones converging outwardly. In later stages of genera the venters are either concave, flat, or rounded, but there is no true keel present, although the venter may become acute in some species at a late stage of growth. The volutions in the earlier stages subsequent to the nepionic are apt to be more or less compressed, the venters assuming early the characters described above, any further sharpening of the venter occurring in later stages. The principal lateral saddles are, so far as known, bifid in the young and show in most groups a tendency to become more or less trifid in later stages. The multiplication of inflections of the suture lines 'increases with the involution of the whorls by growth and they become very numerous in some genera. PLACENTICERAS Meek. The species of this genus could be readily distinguished if it were not for the great range of form in the gerontic stage, which occurs in dwarfed as well as in large specimens, and is continually mistaken for the ephebic stage. The neanic shell is smooth, compressed, with at first a flat and then a concave venter bordered by smooth, entire ridges on the shell and on the casts, but has a stouter volution than later stages. It is, in other words, like the ephebic stage of Protengonoceras in external characters, but the sutures are of the Placenticeran type. Subsequently the ridges become tuberculated, the venter becomes much narrower and the sides also tuberculated and the volutions more compressed. These spines, nodes on the casts, are in three rows, but may be completely absent in some shells, as they are also in some ol...

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Survey, Geological
Editorial: General Books LLC (2012)
ISBN 10: 1235687678 ISBN 13: 9781235687679
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Descripción General Books LLC, 2012. Paperback. Estado de conservación: New. This item is printed on demand. Nº de ref. de la librería INGM9781235687679

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