DR. W. B. CARPENTEE ON ORBITOLITES 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 O. 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. VI.) taken in the radial direction, 



Fig. VI. 



d' 



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, m, m\ m?, m s , m*, 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 l , cP, 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 external 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, m*, 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, a'c', 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, m m ; and this continuity, here transitory, 

 shows itself as the typical character of the chamberlets of the fossil 0. complanata 

 of the Paris basin. But in the existing 0. complanata, a separation comes to be 

 effected, as shown in the succeeding zones, f, f l , f~, f 3 , 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, m m, alternate with them, and are entirely 



MDCCCLXXX1IT. 4 



