GENUS ORBICULINA. 97 



other ; whilst similar multiple bands pass from each series to the rows that stand behind and 

 in front of it, 



141. A comparison of figs. 6 and 10, — of which the former represents the internal 

 arrangement of the cyclical type, and the latter that of the aduncal, — shows that the early- 

 growth of the former, up to the period when the mouth of the spire widens out on both sides, 

 takes place after precisely the same fashion as that of the latter ; after which the aduncal 

 termination is concealed by the extensions of the subsequently formed chambers along its 

 margin, which extensions, having once advanced far enough to meet those advancing in 

 the opposite direction, unite with them, so that each gallery of " chamberlets " thenceforth 

 returns into itself, instead of terminating by two blind extremities. A conception of the 

 probable mode in which the successive additions are made to the shell will help us to under- 

 stand how the one mode of growth may be exchanged for the other, without any departure 

 from the fundamental plan. The extensions of the sarcode-body which form the pseudo- 

 podia issuing from the marginal pores will first coalesce with each other at their bases, so as 

 to form a continuous segment which will lie along the external surface of the last septum ; 

 if this segment were of uniform size along its whole length, the shelly chamber formed by 

 the calcification of its surface-layer would be simple, like that of Peneroplis ; but it is thick- 

 ened in front of each septal pore, and narrowed in the intervals between these thickenings, 

 so as to form a series of secondary segments united by a continuous stolon ; and the shelly 

 envelope moulded upon this will have a corresponding series of " chamberlets " connected by 

 a continuous gallery. Now the tendency of each new segment of the sarcode-body is usually to 

 extend itself along the margin beyond the preceding; but this extension is commonly limited 

 in the early growth of Orhkulina to the inner extremities of the successive segments, which 

 gradually creep round the earlier portion of the spire, whilst their outer extremities terminate 

 abruptly, and may continue so to do through the whole of life. But if these outer extremities 

 of the segments of sarcode should share in the extension, they will creep along the margin 

 of the older part of the shell in the contrary direction, until they meet and coalesce with 

 those of the inner extremities ; and from that time each successive segment of sarcode will 

 be a complete ring which will encircle the entire margin of the disk, and the successive 

 additions made to the shell will be formed upon the cyclical plan. 



142. A larger size and higher development are attained by some fossil forms of 

 OrbicuHna, which there is nevertheless no sufficient ground for regarding as specifically 

 different from the existing type. Under the erroneous designation of Orbitolites Malabarica, 

 Mr. Carter has described (xx) a species of OrbicuUna that abounds in a Tertiary hmestone 

 apparently corresponding with the Nummulitic limestone of the South of Europe ; and of 

 this species I have, through his kindness, had the opportunity of carefully examining several 

 examples. In size it greatly exceeds the existing type, its diameter being sometimes as much 

 as seven or even eight lines ; in general structure, however, it altogether conforms to the 

 description already given, with this one exception, that each surface is formed (as in the 

 complex type of Orbifolitcv) by a layer of shallow chambers that are partially cut ofi" from 

 those of the thick intermediate substance. This difference, however, possesses no greater 

 claim to be accounted a specific distinction in OrbicuHna than it does in OrbiloUtes ; and since, 



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