DR. W. B. CARPENTER ON ORBITOL1TES TENUISSIMA. 
563 
chamber! ets have an elongated form (fig. III., 3, a), and their margins exhibit, even 
! i n the smallest (or youngest) specimens, multiple series of pores (fig. III., 1), in¬ 
dicative of that complicated arrangement of the cavitary system which I described 
minutely in my former Memoir. 
The meaning of that arrangement is best understood by an examination of the 
sarcodic body left after the decalcification of the disks, which are modelled, as it were, 
upon it. The accompanying representation (copied from Plate IV., fig. 4 of my 
Fig. IY. 
Portion of sarcodic body of Orbitolites cornplanata:—a a', b b', the upper and lower annuli 
of two concentric zones; c c, tlie upper layer of superficial sub-segments, and d d, tbe 
lower layer, connected with the annular cords of both zones; e e and e'e', intermediate 
columnar sub-segments of the two zones. 
former Memoir) shows two annular cords, a a, b b', in each annular zone, instead of the 
single cord of 0. duplex; and between these two cords is interposed a series of 
columnar sub-segments, e e, e' e , whose bases and summits (so to speak) are brought 
into continuity by them. It is of the interposed shell-substance that lodges these 
columnar sub-segments, that the thickness of the disk (fig. III., 2) is chiefly made up ; 
and this is obviously in relation with the length of the columns. Between each annular 
cord and the nearest surface of the disk, is a series of sub-segments, cc, dd, which occupy 
the elongated cliainberlets whose partitions are marked externally by radial lines that 
cross the several annuli (fig. III., 3, a), as in O. tenuissima. These partitions, however, 
being complete, the chamberlets have no lateral communication with each other; neither 
do they communicate by means of radial passages with those of the annuli internal and 
external to them. But each has a passage at either end through its own floor, which 
allows a stolon-process to pass from the sub-segment which it lodges to the annular 
cord beneath; each sub-segment being, therefore, in connexion with the two annular 
cords, and forming, as it were, a bridge between one and another, as shown in 
fig. IV. Except through the intermediation of these sub-segments, the annular cords 
of the successive zones have no connexion with each other ; but the intermediate 
columnar sub-segments of each annulus communicate with those of the next by 
