DR. W. B. CARPENTER ON ORBITOL1TES TENUISSIMA. 
565 
other taking place at no fixed stage, but being made sometimes earlier, sometimes 
later, for the most part gradually, but sometimes abruptly, as I indicated in my 
former Memoir (§§ 57, 58). 
I can now trace out more distinctly than before the successive phases of this 
transition; and can show how exactly the fundamental characters of 0. marginalis 
and 0. duplex are reproduced in what may be called the childhood and youth of those 
“ sub-typical ” examples of 0. complanata, which, instead of beginning life on the 
“complex” plan, only attain the more elevated type in adult age. These phases are 
exhibited in the following vertical section (fig. YI.) taken in the radial direction, 
Fig. YI. 
which, though representing them somewhat diagramatically, is true to nature in every 
essential particular. Of the successive zones traversed by the sectional plane between 
the circumambient chamber, c c, which passes round the primordial chamber, p, the 
first five, to, to 1 , to 3 , to 3 , to 4 , are formed exactly on the type of those of 0. marginalis; 
each chamberlet being connected laterally with the other chamberlets of its own zone 
by a single annular canal, ac, and with the chamberlets of the zones internal and 
external to it by the radial stolon-passages, r r. These are succeeded by three zones, 
d, d 1 , d 2 , formed upon the “ duplex ” plan ; each chamberlet having, as in the preceding 
case, but a single annular canal, ac, but communicating with the chamberlet of the 
annulus extenial to it by two oblique radial stolon-passages, as shown at r. Thus, 
then, if the growth of this disk had been checked at the fifth zone, to 4 , its margin 
would have presented the single row of pores characteristic of 0. marginalis; and if 
, at the eighth zone, d 2 , it would have shown the double row characteristic of 0. duplex. 
But in the next zone, e, the annular canals ac, ac, are duplicated, each of them 
sending off a stolon-passage into the next annulus. In this and the succeeding zone, 
e, however, there is no separation between the superficial portions, s s', of the 
chamberlets, and their median portions, to to ; and this continuity, here transitory, 
shows itself as the typical character of the chamberlets of the fossil O. complanata 
of the Paris basin. But in the existing 0. complanata, a separation comes to be 
effected, as shown in the succeeding zones, f, by horizontal extensions of 
the septa i i, that are interposed between the median portions of the chamberlets, so 
as to form the floors of the superficial layers ; while at the same time there is a 
shifting of their relative positions, so that the superficial chamberlets, s s, s' s', instead 
of lying over or under the median portions, to to, alternate with them, and are entirely 
MDCCCLXXX1JT. 4 D 
