78 THE FORAM1NIFERA 



ORDER 10. Nummulitidea. 



Family FUSULTOIDAE. Fusulina (58, p. 1). 



Family POLYSTOMELLIDAE. Polystomella (20, p. 415). 



Family NUMMULITIDAE. Operculina (this article); Amphistegina 

 (51, p. 526) ; Nummulites and Assilina (26, p. 300) ; Heterostegina (this 

 article); Cycloclypeus (10, p. 21, and this article); Orbitoides (31, p. 

 258, and 61, p. 463) ; Miogypsina (60, p. 328). 



Thus the phenomenon of Dimorphism, the occurrence of mem- 

 bers of a species under two forms the megalospheric and micro- 

 spheric is widely spread among the orders of the Foraminifera, 

 and where it is found it affords clear evidence of alternating or 

 recurring generations in the life-history of the species exhibiting it. 



REVIEW OF THE STRUCTURE AND LIFE-HISTORY DISPLAYED 

 IN THE ORDERS OF THE FORAMINIFERA. 



As the attention of the many workers who are occupied with 

 this group is turned to the subject, the list of dimorphic forms 

 will, no doubt, be greatly extended ; but there are indications in 

 the descriptions already published of phases in the life-history of 

 some forms, especially in that borderland occupied by the simpler 

 Foraminifera, which depart in a greater or less degree from those 

 described in Polystomella and Orbitolites ; and we may now take a 

 survey of the features of life -history which have been described 

 in the different groups, and of the more interesting modifications 

 in the form of the test, which, as we have seen, is found to be 

 more or less dependent on the phase of the life -history of the 

 organism which secretes it. 



In chambered tests, in which the walls of the first formed 

 chamber remain unaltered throughout growth, evidence of the 

 mode of origin of the individual, whether as a megalosphere or a 

 microsphere, is furnished by the structure of the test. But in the 

 great majority of the Gromiidea and Astrorhizidea, the tests expand 

 to accommodate the increase by growth (cp. p. 54), and all in- 

 dications of the size of the test when it was first secreted are 

 obliterated. Hence we are deprived in them of part of the evidence 

 on the course of the life-history which we have in other groups, 

 and we must rely on the characters furnished by the soft parts 

 of preserved specimens, or on direct observation of the living 

 animals. 



From the evidence which we have, however, it is not clear 

 that the course of the life-history of some members of these orders 

 is the same as that of the dimorphic Foraminifera above described. 



