GENUS ORBITOLITES. 125 



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 snpcrfcial portions. In either 

 case, the segments of successive zones freely communicate with each other by radiating 

 peduncles of sarcode (also leaving jorwwyes 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. — The extreme freedom with which all the cavities of the shell mutually 

 communicate is a very marked feature in the structure of this type, as in that of Ortjiculina, 

 Alveotina, and Fab id aria ; and shows that the several parts of their animal bodies are far 

 more closely connected into one whole, than they are in those Foraminifera with perforated 

 shells which they most resemble in general plan of conformation. — The addition of 

 new zones, each similar to the last, is a simple matter of rjrowlli ; but the passage from 

 the Simpler to the more Complex plan marks an advance in devetopment ; 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 communications 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 con- 

 nexions that were previously confluent. But in the highest stage of development we find a 

 marked alteration in plan ; for those portions of the columnar segments, 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 intermediate stratum, so as to form a peculiar 

 set of superficial chamberlets ; and these are so equally connected with two zones, as to make 

 it impossible to say that they belong specially to either. — Now we have seen that development 

 may be checked, while r/rowtlL 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 (fossihzed in the Paris basin) developed 

 upon the multiple plan without separation of the superficial chamberlets, while the most 

 complete, in regard alike to mvdtiplication and to differentiation of parts, are only found 

 among the disks at present existing; audit is interesting to observe that some of these 

 present this highest grade of development, while as yet of comparatively minute size. 

 There is scarcely any other type of Animal structure, in which so wide a range of develop- 

 mental variation normally exists. The lower classes of the Vegetable Kingdom, however, 

 especially the group of Fungi, afford abundant examples of it.* 



183. The relation of Orbitolites to Orbiculina is of the most intimate kind. As already 

 mentioned (•[[143), it would not be possible to distinguish with certainty a fragment of the peri- 

 pheral portion of the former from a corresponding fragment of the cychcal type of the latter ; 

 though it is curious to observe that, whilst the differentiation of the superficial from the inter- 

 mediate strata is most complete in the Orbiculince of the early Tertiaries, it is least complete 

 in the Orbitolites of the same epoch ; and conversely, whilst it is least complete in the Orbiculina 



* For a more detailed examination of the reputed species of Orbitolites, see xiii, p. 224. 



