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562 DR. W. B. CARPENTER ON ORBITOLITES TENUISSIMA. 
opening on its external side in a double series. This arrangement, as will presently 
appear, is the first step in the evolution of the “ complex ” form of the Orbitoline 
type. 
Another advance upon 0. marginalis is seen in the more rapid approach of 
O. duplex to the cyclical plan, shown in the abbreviation of the early spiral stage. 
For the “nucleus” of 0. duplex has but a slight eccentricity, and its circumambient 
segment(fig. II., 3, b, b), instead of putting forth but a single stolon-process, gives off 
severalso that, as each of these originates a new sub-segment, a crescentic row of 
sub-segments is at once constituted. The row formed next in succession to this 
almost entirely encircles the milioline nucleus, and the third row generally completes 
the annulus, all further increase in the disk taking place on the cyclical plan. In 
0. duplex, therefore, we have such an abbreviation, not only of the “ milioline ” but 
also of the “orbiculine” stage, that the proper “orbitoline” type is attained at a 
relatively earlier period. 
Fig. III .—Orbitolites complanata. 
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o 
We come, lastly, to that “complex” form—the 0. complanata of Lamarck —in which 
the special peculiarities of the Orbitoline type are most fully displayed. Its disks 
(fig. III., 4) attain not only a much larger diameter, but a relatively greater thickness, 
than those of either of the “simple” species; the annulations which mark their 
surfaces are as complete in their central as in their peripheral portions ; their superficial 
Phil. Trans., 1856, Plate IV., fig. 13, 
