CONCLUDING REMARKS. 565 



tends to surround the whole disk with its mouth or growing margin, and thencefor- 

 ward the growth is cyclical, as in Cycloclypeus and Orbitolites. 



Concluding Remarks. 



ll/. Looking now to the general organization of the five genera which I have 

 described, and to the peculiarities by which I have shown that each is characterized, 

 I think that we are in a position to inquire into the value of the system of classifica- 

 tion which has been erected by M. D'ORBIGNY on the exclusive basis of plan of 

 growth ; on which inquiry, the facts which I have now brought together have an 

 obvious and direct bearing. 



I. The very close physiological relationship which has been shown to exist between 

 Orbitolites and Orbiculina, requires that they should be associated in the same Family, 

 if not in the same Genus. In the classification of M. D'ORBIGNY they are ranked under 

 different Orders (Cyclostegues and Hellcostegues). 



II. In like manner, the close physiological relationship which has been shown to 

 exist between Cycloclypeus and Heterostegina requires that they should be associated 

 in the same Family, if not in the same Genus. In the classification of M. D'ORBIGNY 

 they would be ranked under different Orders (Cyclostegues and Entomostegues). 



III. Again, the strongly-marked physiological difference which has been shown to 

 exist between Orbitolites and Cycloclypeus, would seem to require that they should 

 be separated by the widest possible interval ; yet the system of classification adopted 

 by M. D'ORBIGNY would have forced him to associate them (if he had been acquainted 

 with the last-named type) side by side in the same Order (Cyclostegues}. 



IV. In like manner, the corresponding difference which has been shown to exist 

 between Orbiculina and Heterostegina, would seem to require that they should be 

 separated by the widest possible interval ; yet the system of classification adopted by 

 M. D'ORBIGNY would have forced him to associate them (if he had been acquainted 

 with the real plan of structure of the last-named type) side by side in the same Order 

 (Hellcostegues) . 



V. The doctrine which I base on the foregoing facts, that physiological con- 

 formity in the condition of each individual segment, as indicated by the structure of 

 its shelly investment, is a character of primary importance, whilst the plan of growth, 

 that is, the mode of increase in the number of chambers, is a character of subordi- 

 nate importance, is further borne-out by the following considerations : 



1. In Orbitolites, the general plan being cyclical, the early plan of growth is fre- 



quently spiral. 



2. In Orbiculina, while the early plan of growth is uniformly spiral, and this is 



sometimes continued throughout life, it is very commonly exchanged in 

 adult age for the cyclical. 



3. In Alveollna, whose physiological approximation to Orbitolites and Orbiculina 



is unquestionable, a plan of growth is followed, which differs more from 



4 E 2 



