290 



FAMILY NUMMULINIDA. 



individual chamberlets retaining a pretty close conformity to a constant average. The 

 general plan, in fact, bears a very close resemblance to that of OrbicwKna (% 139) or to that 

 of the spirally-commencing variety of Orhitolites i^ 180). The increase in the number of 

 chamberlets in successive rows always takes place at the inner margin of the spire ; some 

 of those nearest the outer margin dying-out, as it were, without giving origin, to new 

 chamberlets in the next row. This may, I think, be connected with the fact, that there 

 is always a free aperture (e, e) between one row of chamberlets and the next, at the inner 

 margin of each spire (the situation of the aperture in Opercniina), and that the chamber- 

 let which abuts on the preceding whorl is nearly always much larger than the rest of 

 the chamberlets in the same row, and gives origin to two or even three chambers in the 

 next row. Further, it is shown by vertical sections (Fig. XLVII), that the innermost cham- 

 berlets of the whorl are not only deeper but broader, their lateral walls diverging from 

 each other where they are to be continued over the spire they invest. Hence it is pretty 

 obvious that this portion of the whorl is that wherein the most active increase takes place ; 

 and it is here that the marked accession to the number of chamberlets occurs, which tends to 

 carry the later rows around the whole circumference of the spire, and thus to convert it into 

 a disk, as in Orbiailina (1 136). After careful and repeated examination of a great number 

 of sections, I have failed to detect any communication between the adjacent chamberlets of 

 the same row. The chamberlets of successive rows for the most part alternate with each 

 other in position ; and each chamberlet seems, as a rule, to communicate (as shown in 

 Fig. XLVI, a, a) with each of the two chamberlets against which it abuts at either extremity, 



Fig. XLVI. 



More enlarged portion of a section of Eeierostegina, through the median pKine : a a, communications between the 

 chamberlets of successive rows ; b, h, canal-system of the marginal cord. 



in the manner to be more fully described in Cydochjpeus (IF 496). Thus it would appear 

 that each new chamber or row of chamberlets is formed by the independent gemmation of 

 the segments of the preceding row ; and hence it is easy to see why the formation of an 

 incomplete row should be so much more frequent an occurrence in this type, than it is in 

 those similarly-complex types in Avhich a free lateral communication exists between the 

 chamberlets. Each chamberlet is surrounded by its own proper wall, so that the septa which 



