158 Mr. J. J. Lister. Contributions to [June 7, 



Summai'y and Conclusions. 



The following statements relating to the life-history of the For- 

 aminifera appear to be justified : 



1. The species are in a great number of cases dimorphic. The 

 dimorphism has been stated to exist in twenty-three genera, belong- 

 ing to four out of the ten families into which Brady divided the 

 group. 



2. The two forms differ from one another 



(a) In the size of the central chamber. Their difference in this 

 respect is in many cases very marked but may be slight 

 ( Truncatulina) . 



(fe) In the shape and mode of growth of the chambers succeeding 

 the megalosphere and microsphere. 



(c) In the character of their nuclei. In this paper it is shown 

 that in several species the microspheric form has many com- 

 paratively small nuclei, while the megalospheric form has a 

 single large nucleus. 



3. The megalospheric form of a species is much more numerous 

 than the microspheric. 



4. The megalospheric form has been seen to arise in some cases (at 

 least seven genera) as a young individual already invested by a shell, 

 produced in the terminal or peripheral chambers of the parent. 

 While in some cases (Orbitolites) the parent of such megalospheric 

 young was microspheric, in others (Peneroplis, Ch-bitolites) it was 

 megalospheric. 



5. Foraminifera, in certain conditions, give rise to active swarm 

 cells. 



These have been previously recorded in Gromia and Cymbalopora. 

 In Polystomella the protoplasm of a megalospheric form was found 

 broken up into swarm cells of uniform size (isospores), and similar 

 bodies in a flagellated condition have been seen escaping. 



The production of anisospores has been recorded in Miliola 

 (Schneider), and it occurs also in Polystomella as stated above. 



The question has arisen : are the two forms of the Foraminifera 

 distinct from their origin, or is one a modification of the other ? 

 The following reasons may be urged for rejecting the latter 

 hypothesis : 



Among the Miliolidae the plan of growth is often entirely different 

 in the two forms. The hypothesis of modification would in this case 

 require a remodelling of the whole shell. 



If such modification were to occur, various stages in the replace- 

 ment of the megalosphere by small chambers should be found. So 

 far as I am aware such stages have not been found. 



