Memoirs of the Museum of Comparative Zoology at Harvard College Volume 16

Memoirs of the Museum of Comparative Zoology at Harvard College Volume 16

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This historic book may have numerous typos and missing text. Purchasers can usually download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1889 edition. Excerpt: ... Angulatus zone by a full presentation of schlotheimian, caloceran, and vermiceran species, that is, of the entire Plicatus Stock. This stock then entered upon a period of decadence, slight in the Lower Bucklandi, but more marked in the Upper Bucklandi bed. Arnioceras attained its greatest development in the Upper Bucklandi zone and was more persistent in the higher beds than Coroniceras. This last attained its fullest expansion earlier in the Lower Bucklandi beds, and declined rapidly in the Upper Bucklandi, and disappeared altogether in the Obtusus bed. This decline is shown by the geratologous characteristics of the species in the Upper Bucklandi beds, rather than by a less number of forms. Thus Cor. orbiculatum, Gmuendense, trigonatum, and the multicostatus variety of bisulcatnm, are all degenerate species as compared with the forms of the Lower Bucklandi bed. They have more convergent-sided whorls, and these are usually developed at an earlier age. Agassiceras also reached its acme in the Lower Bucklandi bed, but is more persistent, and has some forms in the higher formations. Asteroceras is the only series which attained its acme in the Obtusus zone, and then declined in the Oxynotus zone. The oxynoticeran series reached its maximum in the Oxynotus zone, and, though surviving the changes which attended migration into middle liassic habitats, became extinct in that formation. The schlotheimian series is a highly modified series, composed of involute derivatives, and ceased to exist in the Obtusus bed, but there are a few dwarfed forms in the Oxynotus bed. Caloceras persisted in the highest beds, whereas its highly modified derivative series, Vermiceras, is shorter lived, and less fully represented in the highest beds. Arnioceras more

Product details

  • Paperback | 148 pages
  • 189 x 246 x 8mm | 277g
  • United States
  • English
  • black & white illustrations
  • 1236742850
  • 9781236742858