GENUS ORBICULINA: ORGANIZATION. 551 



them : First, they so closely resemble one another, as to be undistinguishable, in their 

 early condition. Second, they correspond in every particular, so far as regards the 

 structure of their minute parts. Third, the assumption of the cyclical plan of growth 

 does not take-place at any one fixed epoch of development, but may occur at various 

 periods. Fourth, the persistence of the original plan of growth throughout life, can- 

 not be fairly regarded as anything else than an arrest of development, such as we 

 shall presently see to be a common occurrence in Orbiculina, as in Orbitolites, in 

 regard to other particulars (^[ 90). 



88. Turning now from the general plan of growth to the minute structure of the 

 individual parts of Orbiculina, we continue to find a very close conformity to the type 

 of Orbitolites. The texture of the shell is precisely the same ; and it exhibits no 

 other peculiarity than a minute punctation of the superficial layers (Plate XXVIII. 

 fig. 13), which at first suggests the idea of apertures*, but which is found on careful 

 examination of transparent sections (Plate XXIX.fig. 2) to be due to a mere thinning 

 of the shell at certain points, so as to give an appearance of cellular areolation closely 

 resembling what is seen in Orbitolites (Plate VI. fig. 5). 



89. Not having had the good fortune to obtain specimens in which the animal 

 body has been preserved, I cannot speak as confidently on the subject of its conforma- 

 tion, as I could in regard to that of Orbitolites; but a comparison of the features 

 which present themselves in the structure of the calcareous skeleton of the two types, 

 can leave no reasonable doubt that the general arrangement of its segments of 

 sarcode must have been precisely the same. For the conformation of the chambers 

 and passages, as displayed by a horizontal section (Plate XXIX. fig. 1), shows that 

 the soft body must consist of a succession of bands of sarcode, each band swelling at 

 intervals into larger segments ; that the segments of each band usually alternate with 

 those of the bands internal and external to it, so as to be opposite to the intervals 

 between them ; and that the stolons which connect one band with another pass-forth 

 from pores in the intervals between the segments of one, into the segments of the next, 

 those of the outermost band emerging from the margin as pseudopodia. By the 

 coalescence of these a new band would originate, which would become thickened 

 into segments opposite the pores of the preceding, and would give off its own pseudo- 

 podia from the intervals between the segments. 



90. Again, by an examination of the natural margins of Orbiculinas (Plate XXVIII. 

 figs. 6, 7, 18, 19), and by a comparison of these with vertical sections (Plate XXIX. 

 fig. 3), it becomes evident that the same variety exists as in Orbitolites, in regard to 

 the increase of the disk in thickness by the vertical elongation of the segments of 

 sarcode. For some specimens are altogether conformable to the 'simple type' of 

 Orbitolites, in having but a single floor of chambers (so to speak), with a single row 

 of marginal pores; whilst others correspond with the complex type of Orbitolites, in 



* Professor WILLIAMSON (loc. cit.) has described these punctations as perforations for the passage of pseu- 

 dopodia ; but I am quite certain that such is not the case. 



