. Bulletin of the British Museum (Natural History), Geology. U. Aptian - L. Albian L. Albian - U. Albian U. Albian - U. Cenomanian U. Cenomanian only covers those references that relate to the British specimens. In all previous accounts specimens have been referred to the species Orbitolina concava (Lamarck). The form groups of Hofker are distributed as follows: Form group I . . - U. Barremian - U. Aptian Form group II . Form group III. Form group IV. Form group V . As groups III-V are found elsewhere in the stratigraphic interval under consideration a short description of each (based mainly o
. Bulletin of the British Museum (Natural History), Geology. U. Aptian - L. Albian L. Albian - U. Albian U. Albian - U. Cenomanian U. Cenomanian only covers those references that relate to the British specimens. In all previous accounts specimens have been referred to the species Orbitolina concava (Lamarck). The form groups of Hofker are distributed as follows: Form group I . . - U. Barremian - U. Aptian Form group II . Form group III. Form group IV. Form group V . As groups III-V are found elsewhere in the stratigraphic interval under consideration a short description of each (based mainly on Hof ker's work) is given here. These demonstrate, as will be shown in detail later, that the British specimens from the Upper Greensand are completely contained within group IV. Form group III. Proloculus with flat, sometimes concave, distal wall; well-developed epiembryonic chambers with an increasing number of partitions which seldom reach the wall of the prolo- culus. Form group IV. Proloculus very close in appearance to a rounded triangle; epiembryonic cham- bers complex and subdivided into small, interconnected cellules; height of the cellule layer generally about half the height of the deuteroconch. Form group V. Proloculus nearly spherical or arched; outer wall of deuteroconch radial, causing the embryonic apparatus to be more or less globular. epi-embryonic chambers deuteroconch. microgranular layer hyaline wall secfion Fig. 4 Megalospheric embryonic apparatus of Orbitolina lenticularis (Blumenbach). Diagram built up from serial sections through a single test. (x 150.) 20. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original British Museum (Natural History). London : BM(NH)
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