566 DR. CARPENTER'S RESEARCHES ON THE FORAMINIFERA. 



that of either of them, than the plans of the two latter differ from each 



other. 

 4. In Heterostegina, as in Orbiculina, the early plan of growth being uniformly 



spiral, there is a tendency in adult age to the assumption of the cyclical. 

 118. I think myself justified, therefore, by the foregoing comparisons, in asserting 

 that the system of classification proposed by M. D'ORBIGNY is founded on an estima- 

 tion of the value of characters, which is entirely erroneous ; and that any classifica- 

 tion which shall be really natural, must be based on an order of facts relating to the 

 economy of the animal, of which his imperfect methods of observation have left him 

 in entire ignorance*. It is not my intention, in this stage of the inquiry, to propose 

 the erection of any new system ; my sole aim, at present, being to establish the funda- 

 mental principles upon which alone can a natural arrangement be securely built-up. 



EXPLANATION OF THE PLATES. 



PLATE XXVIII. 



Figs. 1-5. Successive stages of growth of the ordinary type of Orbiculina adunca, 



showing the change from the spiral to the cyclical plan of development : 



16 diam. 

 Fig. 6. Edge of a disk of Orbiculina adunca, showing but a single row of apertures, 



as in the simple type of Orbitolites: 50 diam. 

 Fig. 7- Edge of a disk of Orbiculina adunca, showing three rows of apertures, as in 



the complex type of Orbitolites: 50 diarn. 

 Figs. 8-10. Successive stages of growth of the less common type of Orbiculina adunca, 



in which the spiral plan of development is retained throughout life: 16 



diam. 



Fig. 11. Horizontal section of a disk resembling fig. 5 : 16 diam. 

 Fig. 12. Horizontal section of a spiral resembling fig. 10: 16 diam. 

 Figs. 13-16. Portions of the superficies of Orbiculina adunca, showing varieties in the 



surface-markings : 50 diarn. 



Fig. 17. Central portion of a disk of a fossil Orbiculina (Orbitolites Malabaricus, CAR- 

 TER), showing its spiral commencement on the plan of fig. 8: 16 diam. 

 Fig. 18. Marginal portion of a similar disk, showing the investment of the early 



whorl by the later, with the characters of the surfaces and edge : 50 diam. 

 Fig. 19. Marginal portion from a similar fossil disk, which had extended itself like 



fig. 5 : 50 diam. 



* I am constrained to make a similar remark respecting the classification proposed by Professor SCHULTZR 

 (Ober den Organismus der Polythalamien) ; which, although in many respects an improvement upon that of 

 M. D'OBBIGNY, is almost equally far from representing the natural affinities of these organisms, as revealed 

 by minute investigation of the structure of their testa. 



