REPORT ON THE GENUS ORBITOLITES. 21 



diameter of Lamarck's specimens is stated by him at only 2 mm., or about 0"08 inch ; but 

 that of the Fijian specimens ranges to 0"2 inch, or somewhat more. The form of the 

 complete disks (PI. III. fig. 1), when their growth has not been interrupted by injury, 

 is very regularly circular ; and their surface, in all but their central portion, is marked by 

 concentric circles that divide it into annuli having a pretty constant breadth of ^^th 

 inch, each of them marked at regular intervals by dark punctations. When the surface 

 of a peripheral portion of the disk is viewed by reflected light under a higher magnifying 

 power (PI. III. fig. 3), each of these punctations shows itself as a dark spot surrounded 

 by a lighter space, which is often somewhat elevated ; and if the margin of the disk is 

 viewed obliquely, as at a, these circles are seen to be the summits of rows of short 

 cylindrical columns, whose projection gives a slight " fluting " to the edge of the disk. 

 When the edge of the disk is turned directly towards the eye (as at flg. 4, a), a single 

 marginal pore is seen in each of the depressions between the columnar projections: this 

 pore is usually elongated vertically, so as to form a fissure ; and sometimes, when the 

 margin of the disk is unusually thick, as at h, tke fissure is crossed by a shelly bridge, 

 dividing it into two pores. This, however, is not a real duplication of the pores, such as 

 that which is seen at the margin of the species to be presently described (fig. 13). The 

 central portion of the disk (flg. 2) resembles that of Orhitolites tenuissima in the 

 excentricity of its " nucleus," and the incompleteness of the rows of chamberlets first 

 developed around it ; presenting in this stage of its growth exactly that conformity to the 

 spiral plan, which is shown in the third or " orbiculine " stage of the preceding, and 

 the same early approach to the cyclical, which is made by the extension of each 

 new row of chamberlets beyond its predecessor, so that the two ends of the eighth 

 or ninth row meet on the opposite side of the nucleus, forming the first complete 

 annulus. 



Although the " nucleus " itself shows more conformity to the Orbitoline than to the 

 Milioline type, — consisting of a rather large primordial chamber nearly surrounded by a 

 circumambient chamber, — yet its character will be presently seen to be most singularly 

 intermediate between the two. Not unfrequently the " orbiculine " centre of the disk is 

 somewhat thicker than the annular portion by which it is immediately surrounded, so as 

 to form a marked projection from its surface. As new annuli are added-on, however, 

 to the exterior of those first formed, and as the vertical thickness of each is usually 

 rather greater than that of its predecessor, the disk as a whole becomes somewhat 

 biconcave. 



The marginal thickness of the largest disks I have seen of this species is about 0"006 

 inch, or about one thirty-fifth of their diameter. The calcareous lamellse which cover-in 

 the ends of the columnar chamberlets, are so thin as to be translucent, and are very easily 

 abraded ; so that specimens of this type picked out from shore-sands often have the 

 cavities of their chamberlets laid open, as shown in PI. III. fig. 7. The amount of solid 



