GENUS ORBITOLITES: MONSTROSITIES; ESSENTIAL CHARACTERS. 223 



In fact, the manner in which the outer zones of the larger disk envelope the smaller, 

 precisely corresponds with that in which we have seen the new zones originating from 

 the uninjured margin of a mutilated specimen, to extend themselves along its frac- 

 tured edges (^[37.). 



V. Of the Essential Characters of Orbitolites, and of its relations to other Types of 



Structure. 



64. If, now, we seek to determine the essential characters of that type of organi- 

 zation which is known under the designation Orbitolites, we find them to lie in the 

 presence of a series of annuli of sarcode (and of corresponding passages in the shelly 

 disk) arranged concentrically round a nucleus ; each zone in the simpler type con- 

 taining but a single annuhis, so constricted at intervals as to form a series of some- 

 what columnar segments (occupying the cells of the shelly disk), connected with 

 each other by narrow bands of sarcode ; whilst in the more complex type each zone 

 contains two such annuli, including between them a portion of its series of columnar 

 segments, so as to constitute an intermediate stratum, distinct from the superficial 

 portions. In either case, the segments of successive zones freely communicate with 

 each other by radiating bands of sarcode (also leaving passages in the shelly disk), 

 whose normal direction is such as to connect each segment with the two segments 

 that alternate with it in each of the adjacent zones. 



65. Now the addition of new zones, each similar to the last, is a simple matter 

 of growth ; but the passage from the simpler to the more complex plan marks an 

 advance in development; and this advance essentially consists (here as elsewhere) 

 in a progressive differentiation of parts. When, with the vertical extension of the 

 columnar segments, the annular canal subdivides itself into two, the communica- 

 tions between the successive zones no longer come-off, as before, from the annular 

 canal, but from the intermediate portions of the columnar cells ; and instead of the 

 two diverging passages from each cell being in the same plane, they lie in different 

 planes, alternating with each other vertically. Up to this point, we observe little 

 else than a multiplication of parts vertically, as well as horizontally, and a separation 

 of connexions that were previously confluent. But in the highest stage of develop- 

 ment, we find a marked alteration in plan ; for those portions of the columnar seg- 

 ments, which lie between the two annular canals of each zone and the two surfaces 

 of the disk, become completely differentiated from the portions that occupy the inter- 

 mediate stratum, so as to form a peculiar set of superficial cells ; and these are so 

 equally connected with two zones, as to make it impossible to say that they belong 

 specially to either. 



66. Now we have seen that development may be checked, while growth continues, 

 at any period of its progress ; so that we find Orbitolites growing to a considerable 

 size upon the very simplest plan, others still larger formed upon the duplex plan, 

 the largest yet known (fossilized in the Paris basin) developed upon the multiple plan 



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