562 



DR. W. B. CARPENTER ON ORBITOLTTES 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. marginalia 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 O. duplex has but a slight eccentricity, and its circumambient 

 segment (fig. II., 3, b, l>), instead of putting forth but a single stolon-process, gives off 

 several ;* so 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 

 O. 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. lIl.OrMtoJitea complcmata. 



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. 



