DR. W. B. CARPENTER ON ORBITOL1TES TENUISSIMA. 563 



chamberlets have an elongated form (fig. III., 3, a), and their margins exhibit, even 

 in 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. IV. 



Portion of sarcodic body of Orbitolites complanata : a a', b b', the upper and lower annuli 

 of two concentric zones ; c c, the upper layer of superficial sub-segments, and d d, the 

 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, c c, d d, which occupy 

 the elongated chamberlets 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 



