REPORT ON THE GENUS ORBITOLITES. 13 



The " simple " ground-plan is amplified in the " complex " edifice built-up on it by a 

 sort of vertical piling of one storey on another, marked externally by the multiplication 

 of the rows of marginal pores. In so far as this is efieeted by the vertical extension of 

 the sarcodic sub-segments into columns, and by an addition to the number of their 

 annular and radial stolon-processes, the increase may be regarded— like the successive 

 addition of new zones to the periphery — as consisting in growth only. But when, 

 instead of a multiplication of similar parts, we meet with a differentiation in the arrange- 

 ment of these, if not in their character, shown in a separation of the two superficial 

 layers of chamberlets from the intermediate structure (PL VI. fig. 4), by which the 



Fig. 3.— Grouud-plan of shelly Disk of Orbiiolites complanata. 



a. Primordial chamber. 



i, b, Circumambient chamber, a part {h') of which is often partially cut off from the rest by an imperfect partition. 



c, Cj c, Chamberlets of different successive annuli. 



d, Passage from primordial into circumambient chamber. 



e, e, Radial passages from circumambient chamber into chamberlets of first annulus. 

 /,/, Badial passages opening at the margin of the disk as marginal pores. 



" complexity " of the calcareous fabric is considerably increased, this difierentiation 

 must be regarded as an act of development, marking the highest stage of the evolution of 

 the type. 



I have not been able, however, to detect any evidence of local difi'erentiation in the 

 substance of the sarcodic body ; every part of which, even in the most complex forms, 

 seems to have the same character as every other part. A curious evidence of this 

 absence of difi'erentiation is afforded by the fact, that in all the specimens in which the 



